Untitled Flashcards Set

Abalone also known as ear-shells, produce pearls of various hues, such as greens, yellows, blues.

Abrasive material hard, and sometimes brittle, substances used for abrading and polishing purposes, eg, diamond dust, boron carbide (B4C), carborundum , aluminium oxide, garnet, emery, etc. Diamond dust, the most important, is made in various grain sizes, from the finest which is made to 1 micron (0-001 mm.).

Absorption, Differential selective see pleochroism.

Absorption, Selective the absorption of certain colours (wavelengths) from the incident white light when passing through a coloured medium. The colour of the medium results from the mingling of the colours which are not absorbed.

Absorption Spectra the pattern of dark lines or bands seen when light which has passed through a gemstone is examined by a spectroscope.

Accarbaar name applied to black coral. Also Akabar.

Acetone an organic liquid which softens the cellulosic types of plastics and can therefore be of use in their distinction.

Acetylene tetrabromide (tetrabromoethane) a liquid having a formula which may be used as a heavy liquid or as a medium in refractive index determination, S.G. = 2-95, R.I.=1-63.

Achroite colourless tourmaline.

Acicular crystals crystals which have a needle-like form, for example, the crystal inclusions in rutilated quartz (Venus hair stone).

Acid rocks a subdivision of the igneous group of rocks. Generally of light colour, they contain a high content of silica (over 66 per cent). The acid rocks include the granites, syenites and pegmatites.

Actinolated quartz Rock crystal with included crystals of green actinolite.

Actinolite a member of the amphibole family of minerals, actinolite is an end member of the tremolite-actinolite. (Nephrite Jade, part) is a variety of actinolite while the fibrous variety is the asbestos of commerce.

Acute bisectrix the line bisecting the acute angle between the optic axes in biaxial crystals.

Adamantine the term used to describe a a type of luster , that is typical of diamond.

Aaamantine spar name applied to silky brown sapphire.

Adamas ("unconquerable") an ancient name for diamond.

"Adelaide ruby" an undesirable name which has been applied to the almandine garnet found near Adelaide (Australia).

Adularescence the name given to the opalescence seen in moonstone.

Adularia variety of orthoclase feldspar of which moonstone is a gem variety.

"African emerald" a MISNOMER ( COMMERCIAL LABEL ) applied to the green fluorite from South-West Africa. It must not be confused with the true emerald which is also found in South Africa.

Agalmatolite a soft compact material used for carvings. It is a silica-rich variety of pinite, a decomposition mineral with variable composition, but approximating to muscovite (mica). Some agalmatolite is steatite, which see, and the hydrous aluminium silicate in compact form, known as pyrophyllite, is also termed agalmatolite.

AgateAgate is a variety of Chalcedony Quartz and comes in many different color combinations. No two agates are alike. Varieties of agate include Blue, Blue Lace, Crazy Lace, Green, Indian, Moss, Tree, and Wood. Chalcedony (Agate) rates as a “Hardness 7” on the Mohs scale.

AGS American Gem Society – A professional jeweler’s society founded in 1934. AGS has a laboratory which grades diamonds and prepares a diamond grading report.

AGS Diamond Cut Grade The American Gem Society has developed a system for classifying cut quality. The AGS system uses a scale from 0 to 10, where Cut grade 0 is Ideal, 2 as Very Fine and 10 is the lowest grade and quality.

AGTA The American Gem Trade Association is the voice for the colored gemstone industry. Their Code of Ethics holds each Member to a high standard of professional business practices and a higher standard of enhancement disclosure than that required by the Federal Trade Commission.

Alloy An alloy is a mixture of two or more metals.back to top

Amber A delicate, fossilized tree resin. Amber is available in a wide array of colors, the most popular ranging from yellow to orange, mimicking the color of honey touched by the setting sun. Other less common colors of amber include red, green, blue, violet and black. Ranging from transparent to opaque, the finest amber is clear with little or no cloudiness.

Amethyst Amethyst is a member of the quartz family. Amethyst displays a majestic hue of purple, moving from very light to very dark. With purple being the chosen color of royalty, amethyst has long enjoyed popularity. The finest quality of amethyst exhibits a high degree of transparency and a rich deep purple color enhanced by flashes of burgundy or rose. Amethyst are most commonly found in Brazil, Uruguay, Canada and the United States (North Carolina). Amethyst is the birthstone for February.

Anniversary Band Typically given for wedding anniversaries, an anniversary band is a ring that is set with one or more rows of gems, usually diamonds. The diamonds may go completely, three quarters, or half way around the finger.

Appraisal An appraisal is a written estimate of the approximate retail replacement value of the item described. Appraisals can also be used for insurance purposes.back to top

Aquamarine Aquamarine is a member of the beryl family and is related to emerald in composition. The Greeks proclaimed this highly prized, light blue gem aquamarine, because it sparkles like the sea touched by the sun. Found in an array of pastel tones from very light to medium blue, aquamarine is often tinted by a splash of green. The delicate greenish blue of a fine aquamarine conjures up images of dancing light on tropical waters. Most aquamarines come from Brazil. Aquamarine is the birthstone for March.back to top

Agatised coral fossil coral.

Agatised wood see Chalcedony.

AGS abreviation for American Gem Society.

Ahrens prism a calcite prism used for the production of plane polarised light. It is essentially a modification of the Nicol prism designed to obtain a more economical use of calcite.

Akabar name applied to black coral. See also Accarbnar.

Alabaster a massive form of gypsum, see Gypsum.

"Alabaster, Oriental" a MISNOMER ( COMMERCIAL LABEL ) for a stalagmitic variety of calcite characterised by well-marked banding. Another MISNOMER ( COMMERCIAL LABEL ) for this material is "Algerian Onyx".

Alalite see Diopside.

"Alaska black diamond" a MISNOMER ( COMMERCIAL LABEL ) for hematite.

"Alaska diamond" a MISNOMER ( COMMERCIAL LABEL ) for rock crystal.

Alasmoden pearls certain freshwater pearls.

Albertite a jet-like mixture of hydrocarbons R.I. 1-55 ; S.G. 1-097; H. 2.5 Moderately insoluble in most organic solvents.

Albite see Feldspar.

Alcohol (ethyl alcohol) a volatile liquid. May be used for diluting certain heavy liquids and as an immersion liquid (R.I. 1-36).

"Alencon diamond" a MISNOMER ( COMMERCIAL LABEL ) for rock crystal.

Alexandrite see Chrysoberyl.

Alexandrite-like synthetics suitable coloured synthetic corundums and spinels made to imitate the chrysoberyl alexandrite. See Manufactured gems.

"Algerian oynx" a MISNOMER ( COMMERCIAL LABEL ) for a stalagmitic variety of calcite characterised by well-marked banding. Also known as "Oriental Alabaster".

Allochromatic minerals minerals which are perfectly colourless when pure, but may be coloured by impurities, generally a metallic oxide which has no essential part in the chemical composition, or by sub-microscopic particles or inclusions of a coloured mineral, e.g., corundum when pure is colourless (white sapphire), when containing a trace of chromium oxide is red (ruby), titanium oxide giving a blue shade (sapphire), while iron gives greens and yellow shades (green and yellow sapphires). See also Idiochromatic minerals.

Allotropic the name applied to the phenomenon shown by some chemical bodies of assuming different forms, e.g., carbon may form either diamond, charcoal or graphite.

Alluvial deposits deposits of minerals which have been brought down by rivers and are found in their dried-up beds.

Almandine a name applied to the iron-aluminium group of garnets.

Almandine-Pyrope Series the isomorphous garnet series with end-members, pyrope, and almandine. Practically all red garnets belong to this series, being mixtures of pyrope and almandine molecules.

"Almandine spine!" a name applied to the reddish-violet colour of gem spinel.

Almandite see Garnet (Almandine).

"Aloxite" trade name for an abrasive made of synthetically produced aluminium oxide.

"Alundum" trade name for an abrasive made of synthetically produced aluminium oxide.

Amatrix a name, a contraction of American matrix, applied to concretions of variscite in quartz (or chaleedony3. The material is usually cut with the green variscite as centre surrounded by grey, reddish or brownish quartz. S.G. about 2-6 with H from 5 to 6.

Amazonite name applied to the green microcline feldspar, see Feldspar.

"Amazon jade" a MISNOMER ( COMMERCIAL LABEL ) applied to the green microcline feldspar, see Feldspar.

Amazon-stone see Feldspar (Microcline).

Amber a natural resin hydrocarbon. R.I. 1 54; S.G. 1-03 to 1-10; H. 2 to 2.5; Amorphous; Colours, yellow, reddishbrown, bluish, whitish and black; Varieties, Succinite (North German), Roumanite (Roumania), Simetite (Sicily), Burmite (Burma).

Amblygonite R.I. 1-611--1-637, S.G. 3-015 to 3- 033, H. 6. Triclinic. Colourless, yellow and pale mauve. Localities, Brazil, U.S.A.

Ambroid see Pressed amber.

American Gem Society a trade association of jewelers.

"American jade" a MISNOMER ( COMMERCIAL LABEL ) for the massive green variety of idocrase. See Californite.

"American ruby" a MISNOMER ( COMMERCIAL LABEL ) for garnet or rose quartz.

Amethyst violet-coloured quartz, see Quartz.

Amethystine quartz a massive quartz with patchy amethyst colouring. Sometimes used for small carvings.

"Amethyst, Lithia" a MISNOMER ( COMMERCIAL LABEL ) for lilac spodumene.

"Amethyst, Oriental" a MISNOMER ( COMMERCIAL LABEL ) for violet corundum.

Amino plastics name applied to the urea and thiourea in formaldehyde condensation product. They are synthetic resins of the "bakelite" type.

Amorphous (without form) material which has no definite internal structure and havings its properties the same in all directions.

Amphibole the name applied to a group of minerals whose physical and chemical characters serve to link them together, in one family. They are silicates of iron, magnesium, calcium, sometimes sodium (rarely potassium), with or without silicate of aluminium. Nephrite (jade, part), asbestos and hornblende are amphiboles.

Amygdule a rounded or almond-shaped gas cavity in volcanic rocks which later fills with mineral matter, often chalcedony.

Amyl acetate a liquid having a refractive index of 137. The liquid is useful as a test for the cellulosic plastics which soften under its influence.

Analcite R.I. 1-49; S.G. 2-22 -- 2- 29; H. 5-5.5 . Gem material is colourless. Localities, U.S.A., Italy, Czechoslovakia, Japan, Scotland, etc.

Analyser the Nicol prism or "Polaroid" disc which is placed above the objective in the polarising microscope, see also Polariscope.

Anatase R.I. 2-493 -- 2-554; S.G. 3-82 to 3-95; H. 5.5 to 6; Tetragonal; Colours, blue, brown to black; Localities, Switzerland, Brazil.

Andalusite R.I. 1-633 --- 1-644; S.G. 3-1 to 3-2; H. 7 to 7.5; Rhombic; Colours, green, brown and red; Localities, Andalusia (Spain), Ceylon, Madagascar and Brazil. See also Chiastolite.

Andradite see Garnet.

Anglesite R.I. 1-877 --- 1-894; S.G. 6-30 --- 6-39; H. 3. Orthorhombic; White and yellow; Localities, U.S.A. and Scotland.

Angstrom Unit the unit used for the small measurements required in the electro-magnetic spectrum below the infra-red, it is the ten-millionth part of a millimetre.

Anhedral Crystals crystals which do not show good outward form.

Aniline a liquid having R.I. 1-58, useful as an immersion medium.

Anisotropic a term for crystals which exhibit double refraction, i.e., break up a ray of light into two rays which move with different velocities within the crystal. See also, Ordinary ray and Extraordinary ray.

Anomalous double refraction double refraction in isotropic material, as seen by irregular extinction when viewed between crossed nicols. Due to internal strain.

Anorthite see Feldspar.

Antigorite a green serpentine resembling jade.

Antilles pearl a type of "pearl" cut from the shell of the sea snail.

Anyolite a green zoisite rock containing large opaque ruby crystals. Used as an ornamental stone. Found in Tanzania

Apache tears obsidian.

Apatite S.G. 3-15 to 3-22; H. 5; Hexagonal; Colours, blue-green (Moroxite), yellow-green (Asparagus stone), pink, violet, purple, blue (sometimes chatoyant) and colourless; Localities, Saxony, Bohemia, Maine (U.S.A.), Ceylon and Burma.

Aplite a sugary-textured white to grey granite containing silvery muscovite. Used for statuary carving.

Apophyllite S.G. 2-3 to 2-4; H. 4.5 to 5; Tetragonal; Colours, white, grey, yellowish, greenish, and flesh red; Localities, Harz Mountains (Germany), India, Sweden, Tyrol and Transylvania.

Aquamarine see Beryl.

"Aquamarine, Brazilian" a MISNOMER ( COMMERCIAL LABEL ) for blue topaz.

"Aquamarine, Siam" a MISNOMER ( COMMERCIAL LABEL ) for the heat-treated blue zircon.

"Aquamarine, Synthetic" a MISNOMER ( COMMERCIAL LABEL ) for the sea-blue or sea-green colours of synthetic corundum or spinel.

Aqua regia a mixture of the powerful nitric and hydrochloric acids in the proportion of one of nitric to two of hydrochloric used as a solvent for gold.

Aragonite S.G. 2-93; H. 3 to 3.5; Rhombic; Colour, colourless and white; Localities, Aragon (Spain), Bohemia, Sicily, Alston Moor (England), U.S.A., Germany and Hungary. This mineral is the major constituent of pearl.

Aragonite a yellow stalagmitic calcite from South-West Africa.

Arizona Ruby a MISNOMER ( COMMERCIAL LABEL ) for pyrope garnet found in Arizona.

Arizona Spinel a MISNOMER ( COMMERCIAL LABEL ) for red garnet.

Arkansas Diamond a MISNOMER ( COMMERCIAL LABEL ) for rock-crystal.

Artificial resins general name applied to the synthetic products known as plastics. See Plastics.

Artificial stones imitations of gem stones.

Artificial treatment of gemstones anything that has been done by man to enhance the color, clarity or quality of a stone.

Asparagus stone see Apatite.

Asterias stones exhibiting a four, six or twelve-rayed star of light when cut en-cabochon in the correct crystallographic direction. Generally seen in corundums (star rubies and star sapphires with either six or twelve rays) and with rose quartz (generally six rays). Some garnets may also show the effect usually with four rays but if cut in certain directions may show six rays. Star corundums are made synthetically.

Asterism a reflection from fibres or fibrous cavities in a stone, cut en-cabochon with its base parallel to the basal plane of the crystal, in a similar manner to chatoyant stones (see Chatoyancy) but having three sets of fibres crossing at angles of 60 (hence a six-rayed star), or at 90 (a four-rayed star). See also Diasterism and Epiasterism.

Atom a unit which is the smallest part of a chemical element which remains unchanged during all chemical reactions. Although partly composed of electrically charged units such as electrons, protons, etc., atoms as a whole are electrically neutral.

Atomic diamonds see Irradiated diamonds.

Atomic weight the weight of the atom of an element compared with the weight of an atom of oxygen taken as 16. See A table of atomic weights.

Augelite R.I. 1-57 --- 1-588; S.G. near 2-7; H. 5; Monoclinic; Colourless; U.S.A. and Bolivia.

Australite a name applied to the button-shaped pieces of natural glass found in the south of Australia and Tasmania. See also Tektite.

Autoclave a thick walled steel cylinder provided with a removable cover at the top to contain mineral saturated water at fairly high pressures. Used in the hydrothermal method of producing synthetic stones.

Aventurine feldspar see Feldspar (Sunstone).

Aventurine glass a glass made with included copper crystals to produce an imitation of the yellow and red aventurine quartz. An alternative name for this imitation is "Goldstone". Now made in a blue colour.

Aventurine quartz Quartzite with iridescent spangled effect due to green mica or to reddish iron minerals. There is also a grey aventurine quartz which is again due to some form of mica.

Axes, Crystallographic The optical direction of a plane in a stone.

Axe-stone see Nephrite.

Axinite R.I. 1- 67 --- 1-684; S.G. 3-27 to 3-29; H. 6.5 to 7; Triclinic; Colours, brown, honey-yellow and violet; Localities, France and Tasmania, also Cornwall (England), near Ottawa (Canada) and Pennsylvania, New York and Baja California (Mexico).

Axis of symmetry the axis about which rotation of a crystal will cause it to occupy the same position in space more than once in a complete turn. The terms applied to the different classes of axes are: Twice; two-fold, died, half-turn or digonal axis. Three times; three-fold, triad, one-third-turn or trigonal axis. Four times; four-fold, tetrad, quarter-turn or tetragonal axis. Six times; six-fold, hexed, one-sixth-turn or hexagonal axis.

Axis, Optic see Optic axis.

Azure quartz Siderite, see Quartz.

Azurite R.I. 1-730 --- 1- 838; S.G. 3-77 to 3-89; H. 3.5; Monoclinic; Colour, blue; Localities, Russia, Chile, South West Africa, Arizona (U.S.A.), and (:hessy (France). An alternative name is Chessylite.

Azurmalachite an ornamental stone consisting of intimate association of azurite and malachite.

Bacalite name applied to a variety of amber from Lower California, Mexico.

Baguette-cut a modern style of cutting producing a long rectangular-shaped stone with parallel facets; sometimes called batons, both terms being French. See Cuts of stones, "Chart".

Bakelite a synthetic resin sometimes used for imitations of gem materials. Two types (1) a condensation product of phenol (carbolic acid) and formaldehyde. R. I. 1-54 to 1-70 (usually 1-62 to 1-66); S.G. 1-25 to 2-00 (clear types 1-25 to 1-30). (2) Known as Amino Plastics. Urea replaces phenol in the composition. R.I. 1-55 to 1-62; S.G. 1-48 to 1-55.

Bales Ruby a MISNOMER ( COMMERCIAL LABEL ) for the paler types of red spinel, which see.

Ballas an important industrial variety of diamond. The stones are spherical masses of minute diamond crystals arranged more or less radially. They have no well-defined cleavage planes and thus have great resistance to abrasion. Found in Brazil and Africa. Spherical white or greyish diamonds which have cleavage planes are often called "ballas" although they should more correctly be called "bort".

Barite (Barytes) S.G. 4-5; H. 3; Orthorhombic; Colourless, white, yellow, green, red, blue, or brown. World wide occurrence.

Barium titanate This compound has been made synthetically and may at some time be produced as a gemstone. The S.G. is 5-90 and the R.I. 2-40.

Baroque pearls name applied to pearls which are irregular in shape. Also known as Barrok pearls.

Barrok pearls see Baroque pearls.

Basic rocks the name applied to igneous rocks with less than 52 % of SiO,2. See Acid rocks.

Bastite an altered enstatite; S.G. 2-6; H. 3.5 to 4; Colour, leek green; Locality, Harz Mountains.

Bastard-cut a term used for fashioned stones which do not conform to the recognised typical forms, or which show some slight modification from the "pure" forms. The term applies only to those stones which have a regular and symmetrical arrangement of the facets; should they be irregular or haphazard the term Cap-cut is used.

Batons an alternative name for stones cut in the baguette style. See Baguette-cut.

Bayldonite R.I. 195-1-99 ; S.G. 4-35; H. 4.5; Monoclinic; greenish, S.W. Africa, Cornwall (England).

Becke's test a method of refractive index determination.

Beilby layer the name applied to the liquid-like layer of ultramicroscopic depth produced during the polishing of gemstones. It may be described as a local fusion at the minute projecting points on the nearly plane surface and the consequential spreading of a liquid-like layer over the entire surface. The nature of the polish layer may be divided into four groups:

(1) The melting point appears too high for the production of a Beilby layer. The polishing process being merely a fine grinding, e.g., Diamond (and graphite).

(2) The Beilby layer is amorphous (liquid-like) at the moment of formation, but immediately recrystallises in the same orientation as the underlying crystal structure, e.g., Quartz.

(3) In which the Beilby layer recrystallises only on surfaceswhich approximate to important crystal planes (as cleavage surfaces and possible crystal faces), e.g., Calcite and Kyanite.

(4) In which the Beilby layer remains amorphous on all surfaces, e.g., Spinel and Zircon.

Bengal Amethyst a MISNOMER ( COMMERCIAL LABEL ) for purple sapphire.

Benitoite R.I. 1-757 --- 1-804; S.G. 3-64 to 3-65 H. 6.5; Trigonal; Colour, blue; Locality, California.

Benzene a liquid hydrocarbon used for dilution of heavy liquids of organic origin. S.G. 0-88.

Bernat an amber imitation in plastic.

Beryl R.I. 1-560 -- 1-565 to 1-590 --- 1-599 S.G. 2-65 to 2-85; H. 7.5 to 8; Hexagonal; Colours, grass green (Emerald), sea-green and sea-blue (Aquamarine), blue, violet, yellow (golden beryl or Heliodor), pink (Morganite), colourless (Goshenite); Localities, (emerald) Egypt, Colombia, Brazil, Ural Mountains (Russia), S. Africa, Rhodesia, India and Pakistan, Zambia, (aquamarine) Madagascar, Ural Mountains, Brazil and Ceylon, S.W. Africa. Fine blue aquamarines are most commonly the result of heat treatment.

Beryl glass fused beryl which thereby loses its crystalline character and becomes a glass. S.G. 2-41 to 2-49, R.I. 1-50 to 1-52. This glass is sometimes used for green and blue imitation gems.

BerylloIiite R.I. 1-552 --- 1-564 to 1-554 --- 1-566; S.G. 2-80 to 2-85; H. 5.5 to 6; Rhombic; Colour, colourless to pale yellow; Locality, Stoneham, Maine (U.S.A.).

Beryloscope an instrument containing coloured glass dichromatic filters, acting similarly to the Chelsea colour filter.

Bezel that part of a cut stone which lies above the girdle or setting edge. In the brilliant-cut stone it has the table facet and 32 surrounding facets. An alternative name is Crown.

Bezel facets name applied to four of the eight large four-sided facets surrounding the table in the crown of a brilliant-cut stone. An alternative name is Templet.

Biaxial the term used to describe the optical character of anisotropic crystals which have two directions of single refraction; confined to minerals belonging to the rhombic, monoclinic and triclinic crystal systems.

Billitonite a natural glass (tektite) found in Billiton Island, in the Dutch East Indies, and Java, Borneo and Malaya. R.I. 1-51; S.G. 2-45. See Tektite.

Birefringence Double refraction, the amount being measured by the difference between the maximum and minimum refractive indices in an anisotopic mineral.

Bivalve molluscs the name applied to certain species of shellfish whose shelly covering consists of a pair of shells, slightly hollow on the inner side and hinged along one edge. e.g., oyster and mussel.

Biwa pearls non-nucleated cultured pearls from Biwa Lake, Japan.

Black coral a coral consisting mainly of conchiolin and is fished from Pacific waters. The material has a density of 1-34 and has a limited use in jewelry.

Black diamond (a) crystalline diamond, black and virtually opaque, occasionally cut as a gem. (b) Carbonado.

Black dyed opal a type of Australian opal which1 has been stained black to enhance the play of colour.

Black garnet see Garnet (Andradite and Almandine).

Black moonstone name applied to transparent labradorite. See Feldspar.

Black opal a highly-prized opal found in New South Wales, Australia. The dark sandstone in which it is found, being rich in iron, being responsible for the depth of tint.

Black pearls pearls of greyish, brownish or greenish black, found in the Gulf of Mexico and certain of the Pacific Islands. The cause of the colour is not clear but is thought to be due to the nature of the water in which the animal lives. Black pearls are occasionally imitated by polished spheres of haematite. Black clam pearls (q.v.) are not nacreous.

Blende (Zinc-blende) R.I. 2-368 to 2-371; S.G. 4-08 to 4-10; H. 3.5: to 4; Cubic; Colours, yellowish-brown; Localities, Spain and Mexico. Blende has a high dispersion: 0-156.

Blister pearls pearls which have been formed over an irritant which had become cemented to the shell of the mollusc. When removed the pearl has a rough back not covered by nacre which is generally ground flat and covered by the setting. These pearls are mostly of an irregular shape.

Blond shell plain yellow tortoise-shell.

Bloodstone a dark green chalcedony containing spots of red jasper, also called Heliotrope. See Chalcedony.

Blue earth the glauconite sands in which amber is found in the North German deposits.

Blue gold gold of bluish tint induced by the use of steel or iron as an alloy.

Blue ground see Kimberlite.

Blue John massive violet-blue and white banded fluorspar from Derbyshire, hence known as "Derbyshire Spar".

Blue moonstone a MISNOMER ( COMMERCIAL LABEL ) for chalcedony stained blue.

Blue-pearls really a lead-grey in tint, these pearls owe their colour to a large kernel of conchiolin.

Blue-white a name applied to perfection colour in gem diamond. Open to abuse, it is not a satisfactory term.

Boart, Boort, Bort, Bortz the name applied to a cryptocrystalline form of diamond, translucent to opaque and of a dark colour, and sometimes possessing a radial structure. Unlike "ballas" they possess a cleavage. Found in Africa, but a smaller production in Brazil, Venezuela and Guyana. Imperfectly crystallised diamond, lacking cohesive strength, multiple crystals, coated fragments, chips, etc., of various shapes and colours unfit for gems are called boars or Crushing boars, and are used for crushing into diamond powder.

Bohemian diamond a MISNOMER ( COMMERCIAL LABEL ) for rock crystal.

Bohemian garnet the name applied to the pyrope garnet found in Bohemia. These stones cut as roses were prevalent in Victorian jewelry. Many of these Bohemian pyropes show an absorption spectrum more reminiscent of the spinel than the usual spectrum seen in the almandine-pyrope series.

Bohemian ruby a MISNOMER ( COMMERCIAL LABEL ) for red garnet.

Bohemian topez a MISNOMER ( COMMERCIAL LABEL ) for yellow quartz.

Bonamite see Smithsonite.

Bone the hard material composing the skeleton or framework of mammalian animals sometimes used in the simulation of ivory. The material is distinguished from ivory by the difference in structure seen by the microscopical observation of a thin section or peeling.

Bone turquoise see Odontolite.

Boracite R.I. 1-66; S.G. 2-96; H. 7; Cubic; Pale green; Localities, U.S.A. and Germany.

Borazon a cubic form of boron nitride a synthetic product made in the similar way as are synthetic diamonds. Said to be as hard as, or harder than diamond. H; 3-45, but it is only produced as microscopic crystals

Boron carbide an artificially prepared high quality abrasive material. H 9.5 but usually considered to be harder than silicon carbide (carborundum).

Bortz In U.S.A. specially refers to industrial diamonds while Bort is used for crushing purposes.

Botryoidal forms When the surface of a mineral is covered with spherical protuberances due to areas of compact radially arranged fibrous crystals.

Bottle-stone see Moldavite.

Boule the pear-shaped mass of corundum or spinel as it comes from the oxy-hydrogen furnace used in the Verneuil process.

Bowenite a hard variety of serpentine which simulates jade. R.I. near 1-56; S.G. 2-59; H. about 5; has been marketed under the MISNOMER ( COMMERCIAL LABEL ) "New Jade". Some bowenite may be stained.

Brazilianite (Brasilianite) R.I. 1 598-1 617; S.G. 2-94; H. 5.5; Monoclinic; Colour, yellowgreen; Locality, Brazil and the U.S.A.

Brazilian onyx a MISNOMER ( COMMERCIAL LABEL ) for banded calcite.

Brazilian peridot a MISNOMER ( COMMERCIAL LABEL ) for yellow-green tourmaline.

Brazilian ruby a MISNOMER ( COMMERCIAL LABEL ) for pink or "fired" topaz, or pink tourmaline.

Brazilian sapphire a MISNOMER ( COMMERCIAL LABEL ) for either blue topaz or similar colour tourmaline.

Brazilian topaz yellow topaz, see Topuz.

Breccia a mass consisting of broken pieces of rock or mineral with angular edges which has been cemented together by secondary mineralization.

Break facets name applied, in one form of nomenclature, to the 16 small triangular facets on the crown and edging the girdle, and to the 16 similar facets on the pavilion. They are the cross and skill facets. They are sometimes known as the halffacets or halves.

Brewster's angle the angle at which light reflected from a transparent substance exhibits maximum polarisation. This depends upon the R.I. of the substance and R.I. = tan 8.

Brilliant a term used for diamonds cut in the brilliant form.

Brilliant-cut the most important style of cutting for the diamond. The cut consists of 58 facets, a table and 32 facets in the crown and 25 in the base or pavilion of the stone. In the case of large stones the number of facets may be increased. See Cuts of stones Chart.

Brillianteerer the Dutch name applied to the diamond cutter who completes the final facets in a brilliant-cut stone.

Briolette a drop-shaped stone covered with triangular facets. The cut may be considered as a modified double rose.

Bristol diamonds a MISNOMER ( COMMERCIAL LABEL ) for rock crystal, see Quartz.

Brittleness the tendency of a material to easy fracture.

Bromoform S.G. 2-9; R.I. 1-59.

Brom-toluene an organic liquid used for certain refractive index tests. R.I. = 1-55.

Bronzite a mineral closely related to enstatite, which has a chatoyant bronzy lustre.

Brookite R.I. 2-583 --- 2- 705; S.G. 3-87 to 4 08; H. 5.5 to 6; Orthorhombic; Yellowish, hair-brown and reddishbrown. Localities, France, Switzerland and U.S.A.

Brown one of the classifications of gem diamond.

Bruting the actual cutting of one diamond with another diamond, is employed whenever it is necessary to remove diamond rapidly and where finish is unimportant, since only a frosted surface is generated. Formerly a laborious hand operation but is now carried out by an electric eccentric chuck

Bubbles the spherical, oval or tadpole-shape`] inclusions of gas seen in some synthetic stones and most glasses (pastes). They may also be seen in natural and synthetic resins. Also seen in liquid-filled cavities in emerald, topaz, quartz and many other natural minerals.

Buddstone a massive opaque green quartz from South Africa. Used as an ornamental stone.

Burmite name applied to the amber found in Burma.

Button pearls pearls having a rounded top and a flat base.

Bye or Byewater a term used for diamonds tinged with yellow, stones being termed "first bye" or "second bye" according to the amount of yellow color.

Byon the native name for the gem-bearing ground of Upper Burma.

Cabochon a style of stone cutting with a dome-shaped top.

Cacholong a bluish-white porcelain-like variety of opal.

Cacoxenite quartz, usually amethyst, with radiating crystals of cacoxenite.

Cairngorm a brownish-yellow variety of crystalline quartz found in the Cairngorm Mountains, Scotland.

Calamine the name used in some English mineralogical books for the zinc carbonate Smithsonite, which see. In other works this name is used for a zinc silicate.

Calcite R.I. 1-486 --- 1 651 S.G. 2-71 H. 3 Trigonal Colour, colourless and white, sometimes with grey yellow, blue, red, brown or black tints. Marble is a massive calcite, a fibrous form being known as "Satin-spar" and stalagmitic forms with well-marked banding are used for small objects and ornaments and are known under the MISNOMER ( COMMERCIAL LABEL )s: "Algerian onyx", "Mexican onyx" and "Oriental alabaster". Calcite of optical quality, known as Iceland spar, is used in the construction of Nicol prisms and in the dichroscope.

Calcium titanate This compound has been synthesized and may appear on the market as a new gemstone. The material is colourless, orthorhombic, but near cubic, in crystallization. The hardness is 6 to 6l, the density 4-05 and the refractive index 2-40.

Calibre-cut the term applied to stones cut to special shapes. Usually trap cut in style with sharp angular corners. The small square stones used for "eternity" rings are sometimes called calibre.

Californite a massive variety of idocrase which simulates jade. See Idocrase.

Callaite also Callaica, Callaina, and Callais earlier mineralogical names for turquoise, see Turquoise.

Calorescence the term applied to the phenomenon exhibited by certain minerals which, when irradiated with heat rays produce visible light. See also Thermoluminescence and Luminescence.

Cameo the term used to designate those stones, generally composed of two differently coloured layers, in one of which a raised figure or design is cut, while the layer of the second colour forms a background. Agates and certain sea shells are usually the materials used.

Canada balsam a resin obtained from a species of fir and used as a mountant for microscopic specimens. R.I.= 1 53.

Canada moonstone a name applied to the peristerite variety of feldspar. See Peristerite.

Cancrinite R.I. approx. 1-51 S.G. 2-42 to 2-50 H. 5 to 6 Hexagonal Massive opaque yellow or orange. Gem locality is Canada.

Canutillos the name by which good quality emerald crystals are locally known by the miners working the Colombia emerald mines.

Cap-cut a fashioned stone in which the facets are irregular and haphazard. See Bastard-cut.

"Cape emerald" a MISNOMER ( COMMERCIAL LABEL ) which has been applied to the mineral prehnite found in South Africa. See Prehnite.

"Cape ruby" a MISNOMER ( COMMERCIAL LABEL ) for pyrope garnet found in association with diamond in South Africa.

Cape stones a classification of gem diamond sub-divided into: fine silver Cape, silver Cape light Cape, Cape, dark Cape. This group is classified next in order to blue-whites and whites.

Carat (precious metals)a term used to express the fineness of gold used in jewelry. It may be better understood as a twenty-fourth part, thus 9 carat gold contains 9 parts of pure gold and 15 parts of alloy, likewise 22 carat gold contains 22 parts of pure gold and 2 parts of alloy. The term is sometimes spelled Karat.

Carat weight the unit of weight for diamonds and gemstones. It is defined as one-fifth of a gram (200 milligrams=0 200 gram). It became legal standard on the 1st of April, 1914, and was and frequently still is, known as the metric carat. The old London carat weighed 0.20530 gram, was not a legal standard and did not conform to the carat weight in other parts of the world, which varied in different countries from 0-1885 gram to 0-2135 gram.

Carbon

(1) see Carbonado.

(2) black inclusions in diamond, often referred to as carbon marks.

Carbonado an opaque, black, tough and compact variety of diamond found in Brazil, specific gravity being between 2-9 to 3-5. It is used for drilling Bits for deep boring and for slow running abrasive wheels.

Carbon disulphide A liquid R.I. 1 63.

Carborundum H. 9.5 S.G. = 3-17 a synthetic product made by heating coke and sand in an electric furnace. Used as an abrasive.

Carbuncle a name for almandine garnet which has been cut en-cabochon.

Carnelian see Cornelian.

Cascalho the native name for the diamond-bearing gravel of Brazil.

Casein a synthetic substance made from the albumen of milk and used occasionally as an imitation of amber and tortoiseshell and some ornamental stones. R.I. 1-55 to 1-56 S.G. 1-32 to 1-39 (usually 1-32 to 1-34.

Cassia oil a vegetable oil akin to cinnamon oil. R.I.= 1 60.

Cassiterite R.I. 1-997 --- 2- 093 S.G. 6-8 to 7-1 H. 6 to 7 Tetragonal Colours, red, brown, black and yellow Localities, Cornwall, Bohemia and Saxony.

Castor oil a pale yellow oil obtained from the seeds of ricinus communis and used as an immersion medium in certain refractive tests. R.I.= 1-48.

Cat's-eyes stones which, when cut en-cabochon, show a wavy changeable band of light across the dome. The phenomenon is known as "chatoyancy", and is often observed in quartz, chrysoberyl and tourmaline. See also Chatoyancy, Hawk'seye and Tiger's-eye.

Cedar wood oil a vegetable oil used in immersion refractive index tests. R.I. = 1-51.

Celestine (Celestite) R.I. 1-62 --- 1-63 S.G. 3-97 to 4-00 H. 3 to 3.5 Orthorhombic Colourless to bluish Locality U.S.A.

Celluloid a thermo-plastic material made from a nitrocellulose base, sometimes used as an imitation of amber, etc. Two types:

A-Ordinary celluloid (cellulose nitrate). R.I. 1-495 to 1-51 S.G. 1-36 to 1-80 (usually in the clear types 1-36 to 1-42.

B-Safety celluloid (cellulose acetate). R.I. 1490 to 1505 S.G. 1-29 to 1-80 (usually 1-29 to 1-40)

Cerussite R.I. 1-80 --- 2-1 S.G. 6-5 H. 3.5 Orthorhombic White, grey, green, blue and black. World wide occurrence.

Ceylon diamond a MISNOMER ( COMMERCIAL LABEL ) for colourless zircon.

Ceylonese chrysolite a MISNOMER ( COMMERCIAL LABEL ) for greenish-yellow tourmaline.

Ceylonese peridot a MISNOMER ( COMMERCIAL LABEL ) for yellowish-green tourmaline.

Ceylonite a dark green, almost opaque spinel, rich in iron sometimes used in jewelry. An alternative name for this variety is Pleonaste, see Spinel.

Ceylon opal a MISNOMER ( COMMERCIAL LABEL ) for moonstone.

C.G (Certified Gemmologist) suffix used by gemmologists who have qualified by the examination of the Gemmological Institute of America.

Chalcedony a micro-crystalline variety of quartz , with the following varieties:Chalcedony common translucent with a white or bluish colour, green when containing chromium.

Cornelian translucent flesh-red.

Sard brownish red.

Chrysoprase translucent apple green.

1-Plasma dark green with white or yellowish spots.

2-Bloodstone or Heliotrope dark green with scattered spots of red jasper.

3-Agates chalcedony where the colour is variously distributed, generally in parallel layers.

4-Banded agate colours in parallel bands.

5-Eyed-agate bands having a circular arrangement.

6-Fortification agate bands are angularly arranged.

7-Moss agate or Mocha stone containing dendritic inclusions. Agatised wood chalcedonic pseudomorph after wood.

8-Onyx similar to agate except that the bands are straight. Cameos are usually cut from these. Onyx, like all chalcedonies, can be stained, the black onyx nearly always has been so treated.

9-Sardonyx as onyx except that instead of the colours being black and white they are brownish red and white.

10 Jasper an impure variety of micro-crystalline quartz, opaque reds and browns also greyish blue and greens. Riband jasper is striped.

11 Hornstone a grey impure form which is sometimes stained to imitate lapis-lazuli. (This shows red under the colour filter whereas true lazurite does not.)

The refractive indices and specific gravity of Chalcedony are but slightly lower than for Quartz.

Chatham emerald (Chatham Cultured Emerald: Chatham Created Emerald) names applied at various times to an American synthetic emerald, grown by C. F. Chatham.

Chatons paste (glass) stones backed with a reflecting foil.

Chatoyancy (cat's-eye effect) is due to the reflection of light from fine fibres or fibrous cavities within the stone. The wavy band of light seen across the stone being at right angles to the direction of the fibres. To show this best stones must be cut en-cabochon. See also Asterism.

Chatter marks see Fire marks.

Chelsea colour filter

Chemical composition the composition of a molecule of a substance, which may be an element or a combination of different elements in quantities which must obey certain definite chemical laws.

Chemical composition of gem minerals See Chart

Chemical elements matter composed of only one chemical type, and which thus cannot be decomposed into simpler substances by chemical means. See Chart

Chessylite see Azurite.

Chiastolite a variety of andalusite (which see) containing carbonaceous inclusions in the form of a cross

Chicot pearls an alternative name for blister pearls. See Blister pearls.

Chlorastrolite a greenish fibrous mineral related to prehnite. S.G. 3-2 H. 5 to 6 Colour, chatoyant green and white Locality, Lake Superior (U.S.A.).

Chloromelanite a dark green nearly black ferruginous variety of jadeite. S.G. = 3-4. H. - 6.5 to 7.

Chlorospinel see Spinel.

Chondronite R.I. 1-60 --- 1-63 S.G. 31 H. 6.5 Monoclinic Yellow, red, brown Localities, Sweden and U.S.A.

Chrome chalcedony green chalcedony coloured with chromium. Found in Rhodesia. Not to be confused with chryosprase which it resembles.

Chrome diopside a bright green diopside found in association with diamond in South Africa.

Chromite S.G. 4-3 to 4-6 H. 5.5 Cubic, Colour iron-black to brownish-black Localities, U.S.A., etc.

Chromium oxide a green powder used as a polishing agent, See Green rouge.

Chryselephantine the name applied to objects of art composed or overlaid, partly with gold and partly with ivory.

Chrysoberyl R.I. 1-742 --- 1-749 to 1-75 --- 1-757 S.G. 3-68 to 3-78 H. 8.5 Rhombic Colour, greenish-yellow, greenish chatoyant (Cymophane or Cat's-eye), emeraldgreen in daylight and red in artificial light (Alexandrite) Localities, Brazil, Ceylon, Ural Mountains (Russia), and Rhodesia.

Chrysocolla a hydrous copper silicate R.I. 1-50 S.G. 2-1 to 2-2 H. 2 to 4 Amorphous Colour, green and greenish-blue, Localities, Ural Mountains (Russia), Chile and Arizona (U.S.A.). Often impregnating) quartz.

Chrysolite an ancient name applied to various kinds of yellow and greenish-yellow stones. A name which is better discontinued.

Chrysolite, Water a MISNOMER ( COMMERCIAL LABEL ) for moldavite.

Chrysoprase apple-green chalcedony.

Cinnamon oil an aromatic oil used in certain refractive index tests. R.I. = 1-59.

Cinnamon-stone brownish-red hessonite garnet.

Circular polarisation the peculiar property of quartz, among gemstones, of rotating the plane of polarisation of a ray of light passing parallel to the optic axis, and showing an interference figure in convergent polarised light, in which the arms do not meet at the centre, the four arms stopping at the innermost ring.

Citine yellow quartz, see Quartz.

Clam pearls pearls obtained from the clams such as the quahog or hard clam (Venus mercenaria) and the giant clam (Tridacna gigas). Density varies from 2-20 to 2-66.

Cleavage the tendency of a crystallised mineral to break along certain definite directions producing more or less smooth surfaces.

Cleavage name applied to diamond crystals showing many flaws, or to broken fragments of crystals.

Cleaving the method of dividing a diamond crystal into two or more pieces by splitting the stone through the grain (cleavage direction).

Clerici's solution

Clmozeisite An epidote containing less than 10% of the iron molecule R.I. 1-72 --- 1-734 S.G. near to 3- 37. It is a lighter green than epidote.

Close goods are whole diamond crystals which contain no flaws.

Clove oil an aromatic oil used in certain refractive index tests. R.I. = 1-54.

Coated stones diamond crystals having a coat of green, brown or yellow colouring removable by cutting.

Cobalt glass a glass coloured blue by cobalt oxide. Often used in the production of imitation gems, this glass is characterised by a typical absorption spectrum.

Cobaltite S.G. 60 to 64 H. 5.5 Cubic Colour, silver-white Localities, Scandinavia, U.S.A. and England.

Cohesion the name given to the force of attraction existing between the molecules of one and the same body in consequence of which they offer a resistance to any influence tending to separate them.

Colemanite R.I. 1-58 --- 1-61 S.G. 2-42 H. 4.5 Monoclinic Colourless and white Locality, U.S.A.

Collet, Collette alternative names for culet which see.

Collimator the lens system in certain optical apparatus used to parallelise the incident light rays. See also Spectrometer.

Colloid the term applied to a liquid or solid compound of one substance in fine particles of ultra-microscopic size diffused through another.

Colorado jade a MISNOMER ( COMMERCIAL LABEL ) for amazonite. See Feldspar.

Colorado ruby a MISNOMER ( COMMERCIAL LABEL ) for pyrope garnet found in Colorado, U.S.A.

Colorimeter, Diamond see Diamond colorimeter.

Coloriscope a Swiss instrument for the colour grading of diamonds.

Colour dispersion See Chart

Coloured diamonds diamonds having a different shade of colour, termed Fancy diamonds, such as red, pink, blue, mauve, green, canary-yellow and brown. (See note on irradiated diamonds)

Colour filters coloured films or glasses used to filter out certain colours of the spectrum.

Colours of gemstones

Composite stones

Conchoidal fracture --- 2-6 S.G. 5-9 to 6-1 H. 2.5-3 Monoclinic Hyacinth red Localities, Tasmania, Russia, Romania, Brazil and U.S.A.

Crocus (Crocus martis) a polishing powder produced from an iron oxide.

Cross-cut See scissors cut.

Cross facets the name applied to eight of the small three-sided facets around the girdle edge on the crown, which in the case of a modern circular stone have the same size and shape as the eight skill facets adjacent. In older oval-shaped stones these facets are the eight larger of the 16 edge facets. In modern nomenclature the eight cross and the eight skill facets are combined as 16 half or break facets. An alternative name for cross facets is skew facets.

Cross stones fancy name for the twinned crystals of staurolite. Also known as "Fairy stone".

Cross work the name applied to the first operation in grinding a brilliant-cut stone, consisting in grinding the table and four main facets

Crown that part of a cut stone which lies above the girdle, or setting edge. In the brilliant-cut stone it has the table facet and 32 surrounding facets. An alternative name is the bezel.

Crown glass a classification or family of glasses which do not include lead oxide in their composition. In general they have lower constants than for the more highly dispersive "flint" or "lead" glasses.

Crypto-crystalline the term used to describe material made up of an aggregate of sub-microscopic crystals.

Crystals solids possessing a certain definite internal atomic structure, which is identical in the case of crystals of any one species. This definite arrangement directly influences the geometrical form and the physical and optical properties.

Crystal axes imaginary lines of reference running through the ideal crystal and intersecting in the centre at a fixed point, termed the origin. They are reference lines from which can be measured the relative positions of the various faces.

Crystal faces the flat surfaces of geometrical outline which form the bounding surfaces of crystals. In the case of some diamonds these surfaces are curved.

Crystal lattice the three dimensional array of points (atomic positions) in space at which the pattern repeats itself. There are only 14 possible variations of the crystal lattice.

Crystalline material any material which shows by physical and optical means the regular arrangement of its internal atoms.

Crystallography the study of crystals and their structure.

Crystal, Rock see Quartz.

Crystal systems see Chart.

Crystolon trade name for an abrasive made of silicon carbide.

Cube a solid of six square faces with all its angles right-angles. The fundamental crystal form of the cubic system.

Cubic system one of the crystal systems, see Chart

Cubo octahedron a crystal form combining the cube and the octahedron.

Culet the small facet at the base of the pavilion of a brilliantcut stone parallel to the table facet. Its main function is to prevent splintering but it is often omitted in modern cut stones.

Cullinan diamond also known as the Star of Africa, is the largest diamond ever found. Found at the Premier mine in the Transvaal on 25th January, 1905, the rough stone weighed 3,106 carats. From this magnificent stone two important diamonds were cut one, a pendeloque brilliant weighing 530-2 carats is the largest cut diamond in the world and is mounted in the Royal Sceptre of the British Regalia the other, a square brilliant weighing 317-4 carats is set in the Imperial State Crown. These two stones are known respectively as Star of Africa No. 1, and Star of Africa No 2.

Cultured pearl a pearl produced by the insertion in the pearl oyster of an artificial nucleus, usually mother-of-pearl, and the deposition of nacre thereon by the mollusc.

Cuprite R.I. 2 85 S.G. 5 85 to 6 15 H. 4 Cubic Red Widespread occurrence.

Curvette see Chart.

Cut-Cornered triangle the name applied to a trap-cut stone in which the outline is that of a triangle with two of the corners bevelled off. See Cuts of stones, see Chart.

Cuts of Stones see Chart

Cuttable rough a name which has been applied to all diamonds suitable for manufacturing into ornamental or gem stones.

Cutting the process of cutting gemstones on revolving diamond charged grinding wheels.

Cyanite alternative spelling for kyanite, which see.

Cyclotroned diamonds Diamonds which are coloured green by bombardment with atomic particles which have been accelerated to a high speed by a cyclotron. After subsequent heat treatment the stones become yellow or brown. The colouration is only skin deep and the stones are not radioactive.

Cymophane see Chrysoberyl.

Cyprine see Idocrase.

Cyst pearls pearls formed within the tissue of the mollusc itself. These pearls are the most perfect.

Dallasite a name applied to the green and white jasper from Vancouver Island, British Columbia.

Danburite R.I. 1-63 --- 1-636; S.G. 3-00; H. 7; Rhombic; Colour, colourless and yellow; Localities, Madagascar, Japan, Burma and Switzerland.

Dark brown a classification of gem diamond.

Dark cape a classification of gem diamond. See Cape stones.

Datolite R.I. 1-625 --- 1- 669; S.G. 2-9 to 3-0; H. 5 to 5.5; Monoclinic; Colour, whitish, yellowish, colourless, reddish, greenish, brownish and mottled; Localities, U.S.A.

Deer horn the horn or antler of certain of the deer family has been used instead of ivory for small carvings, particularly for the netsukes of Japan. S.G. 1-6 to 1-85.

Delawarite a name applied to the aventurine feldspar found in Delaware Co., Pennsylvania, U.S.A.

Demantoid see Garnet.

Dendritic the tree or fern-like form assumed by some minerals, particularly when they are inclusions in others, such as the dark pigmenting minerals in quartz producing moss agates.

Density the comparison of the weight of a given volume of a substance with the weight of a similar volume of another substance used as a standard, see also Specific gravity,

Density correction tables see Chart

Derbyshire spar see Fluorspar and Blue John.

Deviation, minimum see Refractive index Chart.

Diakon see Perspex and Plastics, See Chart.

Diamante a term used in jewelry for paste set articles.

Diamantine trade name for ar abrasive made of aluminium oxide. Note, there is no diamond content.

Diamonair name applied to the synthetically produced yttrium aluminium garnet.

Diamond R.I. 2-417 to 2-420; S.G. 3-51 to 3-53; H. 10; Cubic, with a perfect cleavage parallel to the faces of the octahedron; Colour, colourless and pale tints of yellow, red, pink, green, blue, also brown. Some green diamonds have been artificially tinted by radium emanations. There are two distinct types of diamond;

(1) to which the majority of stones belong, which exhibits complete absorption beyond 3,000 A;

(2) the rarer "transparent" type, which transmits light down to 2,250 A. The dispersion (B-G)= 0 044; Occurrence is from alluvial deposits and from pipes (volcanic?) in South Africa; Localities, India, Brazil, South Africa, South West Africa, Tanzania, West Africa, Australia, Guyana, Congo, Ghana, Borneo and U.S.S.R. See Radiumtreated diamonds, Irradiated diamonds, Coloured diamonds, Industrial diamonds.

Diamondlite an American microscope used for the grading of diamonds.

Diamond colorimeter an American instrument for the colour grading of diamonds. The Diamolite is a similar instrument.

Diamond dust see Diamond powder.

Diamond gauge devices of assistance in the estimation of the weight of a mounted diamond. They are of two general types.

(a) A stencil gauge consisting of a thin sheet of metal or celluloid (the metal types often being in the form of folding leaves) in which are a series of differently sized circular apertures, each of which has a diameter agreeing with the diameter of a correctly fashioned diamond of given weight. The gauge is placed over the stone to be estimated and the aperture which just fits over the girdle of the stone gives the approximate weight of the diamond, each hole being marked with its value in carats or decimals of a carat.

(b) a pair of spring calipers with the moving arm fitted with a pointer which moves over a scale of numbers. The diameter of the stone is first measured and then the depth. The readings obtained are looked up in a book of tables supplied with the instrument and the approximate value read from them. It is considerably more accurate than the stencil gauge. See a]so Moe's diamond gauge.

Diamond point the relation of the table of a cut diamond to the underlying regular octahedron. It is said to be four-point, if the table be cut parallel to the face of the cube, that is across the corner of the octahedron so that the resulting section is square; three-point, if the table be parallel to an octahedral face; and two-point, if the table be parallel to the face of the rhombic dodecahedron and therefore to an edge of the octahedron, while equally inclined to its two faces meeting in that edge.

Diamond point the earliest form of diamond fashioning, being merely the polishing of the octahedral faces of the crystal to a regular shape.

Diamond powder boars and remainders from diamond cleaving and bruting reduced to powder by various mechanical methods (usually by crushing in an iron mortar) and then segregated into micron size groups by either air sifting, centrifuge, elutriation or settling in oil. Finest material commercially produced has a grit size of 1 micron (1 micron = O-OOl mm.). Diamond powder is used for grinding and polishing diamonds and other hard stones and for diamond saws and other industrial purposes. See Diamond saw.

Diamond saw a disc of phosphor bronze about 2" in diameter and about 07 to 12 millimeter in thickness, with a thickened periphery charged with diamond powder.

Diamondscope an American instrument consisting of a binocular microscope with a specially designed dark-field illuminator used for observing imperfections and internal features in diamonds and coloured stones.

Diamonds, Industrial certain types of diamonds, generally those unsuited for gemstone production, which have applications in modern industrial and engineering practice. They are used for:

......Diamond powder

......Glaziers diamonds

......Indenter hardness testers

......Truing tools for grinding wheels

......Turning tools,

......Wire drawing dies, etc.

......See also Ballas, Boart, Carbonardo, Diamond powder and Diamond saws.

Diaphaneity see Transparency.

Diasterism a star effect which is best seen when light is transmitted through the specimen. Phlogophite mica and some rose quartz shows this effect.

Diatomite a polishing powder produced from a soft silica composed of minute plant skeletons or diatoms. Sometimes known as "Fossil tripoli", it should not be confused with tripoli.

Dichroism the differential selective absorption of light seen in some doubly refractive stones. See Chart.

Dichroite see Iolite.

Dichroscope an instrument comprising a suitably cut rhomb of Iceland-spar and a lens system in a short tube, used for viewing the effects of dichroism.

Differential Selective absorption of light see Pleochroism, see Chart1

Diffraction Grating a series of fine lines ruled on glass or metal and used to produce spectra. They are used in some types of spectroscopes.

Diffraction of light the breaking up of white light into the spectrum colours when it passes through a narrow aperture. It is a special case of interference of light.

Diffusion column a tube containing two heavy liquids, one being less dense than the other, allowed to diffuse together so that the resultant liquid varies in density from top to bottom. Stones having specific gravities between the limits of the liquid will take up positions at differing levels. It is a method for quickly ascertaining the density of stones of slightly differing specific gravity such as stones of different colour of the same species.

Diffusion melt similar topux diffusion except the chemical compounds used are usually in separate layers and the crystallization occurs after diffusion of the chemicals when the mass is fused.

Dimethylaniline a liquid having an application in certain refractive index determinations. R.I.= 1 56.

Dimetric system alternative name for the Tetragonal system. Dimorphism; term applied to the case where two minerals have the same composition but a different crystal structure.

Dinny bone (Dinosaur bone) fossil dinosaur bone used as an ornamental stone.

Diopside R.I. 1-67 --- 1-70; S.G. 3-20 to 3-32; H. 5 to 6; Monoclinic; Colour, green; Localities, Italy and the U.S.A. Alternative names, Alalite and Malacolite. A massive dark violet-blue variety from Piedmont known as "Violane", is used as an ornamental stone.

Dioptase R.I. 1-655 --- 1-708; S.G. 3-3; H. 5; Trigonal; Colour, emerald-green; Localities, Siberia, Chile, Congo.

Dispersion the breaking up of white light into the spectrum colours when a ray passes across two inclined faces of the stone. In a gemstone it is known as "fire" See Chart 142.

Dodecahedron a geometrical solid having twelve faces. The rhombic dodecahedron has twelve lozenge-shaped faces and is a form found in the cubic system.

Dop

(1) Solder: A brass cup with a malleable copper stem filled with lead-tin solder o which the diamond is set in order to cut and polish the facets.

(2) Mechanical: A holder in which the diamond is held between steel claws tightened by a screw, enabling the stone to be automatically adjusted without re-setting.

Double refraction the effect caused by all crystals, except those of the cubic system, of splitting a ray of light which passes into them into two rays, which travel with differ

Double rose the name applied to a cut stone of spherical shape covered all over with triangular facets; it may be assumed to be two rose-cut stones base to base.

Doublets

(1) Composite stones,

(2) Spectroscopic, close pairs of lines seen in emission and absorption spectra.

Dravite Brown tourmaline.

Drop pearls pearls having a drop or pear-shape. Sometimes called "pear-eyes".

Dullam the concentrated gem gravel (illam) of Ceylon which contains the gem minerals.

Dumortierite R.I. 1678-1689 ; S.G. 3 26 to 3 36; H. 7; Rhombic; Colour, blue-violet; Locality, California, Arizona and Nevada.

Durability the resistance a stone possesses to forces which tend to destroy its lustre and polish. These forces may be physical or chemical.

Durangite R.I. 1- 66 --- 1 71; S.G. 3-97 to 4-07; H. 5; Monoclinic; Orange-red. From Mexico.

Dust, diamond see Diamond powder.

Dust diamond, assorted diamonds, usually 60 per carat down to 150 per carat in size, and having a degree of brilliancy so that they can be used for ornamentation of cheap jewelry without further polishing. A portion of this material is used industrially. Not to be confused with diamond dust.

Dwt, Pwt or pennyweight 24 grains, 0.05 ounces, metric 0.0648 grams.

Dyed stones a number of gem materials are sufliciently porous to readily take colouring agents and such treatment is commercially carried out. Chalcedony which is often inorganically stained various colours. Other stones sometimes dyed are turquoise jadeite, opal, serpentine, and alabaster. If the material has been stained and this can be proved, the fact should be disclosed, but in some cases this may not be possible, and in these cases, particularly the black and red colours of chalcedony, the names onyx, cornelian and sard would be commercially acceptable.

Ear-shells see Abalone.

Egeran a name applied to the variety of idocrase found at Eger, Hungary. See Idocrase.

"Egyptian alabaster" name applied to a banded calcite. The alabaster of the ancients being calcite. The modern alabaster is massive gypsum.

Egyptian jasper jasper in which the colours run in zones.

Egyptian marble a marble stained black by bitumen and veined yellow with dolomite. It is usually obtained from Italy.

Eight-cut a simple modification of the brilliant-cut used for small diamonds. The table being surrounded by eight foursided faces. See Cuts of stone, see Chart 186.

Eilat stone a blue to green mottled stone consisting of a mixture of chrysocolla and other copper minerals. Found in Israel.

Ekamte a metamict calcium thorium silicate; R.I. 1-60; S.G. 3-28; H. 6.5. Stones have a green colour, are singly refractive and are radio-active. Only known source is Ceylon.

Elaeolite a massive variety of the mineral nepheline R.I. 1-538 --- 1-542; H. 5 to 6; S.G. 2-55 to 2-65; Hexagonal; Colours, green, red, brown, blue and grey.

Elath stone same as Eilat stone, which see.

Electron volt Unit of energy used in nuclear physics. It is used by physicists rather than wavelength to which it has a definite relation.

Ele Emery an intimate mixture of granular corundum, magnetite and some haematite. Often used for abrasive purposes, the grain is found to break down under pressure, so that new cutting edges are constantly present. The material is best suited for polishing purposes. ctro-conductivity; when a substance, whether mineral or not, after being held between two electrodes at different electrical potentials allows current to pass the substance is said to be electro-conductive. The test has a value with blue diamonds, for, as far as has been found, all natural blue diamonds conduct electricity as they are of the Type IIb, while the diamonds artificially coloured by electron bombardment are not electro-conducting.

Electro-magnetic spectrum the term used for the range of wave-lengths of radiant energy from the long waves of wireless to the extremely short wave-lengths of the cosmic radiations. Below the wireless waves come the infra-red or heat rays, light rays, ultra-violet rays, X-rays, the gamma rays of radium and the cosmic rays.

Electroned diamonds diamonds coloured an aquamarine blue colour by treatment with high-speed electrons from a Van de Graaf generator. The colouring is only skin deep and the stones are not radio-active, nor are they electro-conductive.

Emerald grass-green beryl, which see.

"Emerald, African" green fluorspar.

"Emerald, Brazilian" a MISNOMER ( COMMERCIAL LABEL ) for green tourmaline.

Emerald coated beryl cut and polished aquamarine or yellow beryl upon which a coating of synthetic emerald has been deposited. See Emerita.

"Emerald, Evening" a MISNOMER ( COMMERCIAL LABEL ) for peridot.

"Emerald matrix" a MISNOMER ( COMMERCIAL LABEL ) for green fluorspar.

"Emerald, Oriental" a MISNOMER ( COMMERCIAL LABEL ) for green corundum.

"Emerald, Scientific" may be synthetic corundum or spinel, a beryl glass or just paste.

Emerald, Spanish" a MISNOMER ( COMMERCIAL LABEL ) for green glass.

Emerald, Synthetic see Manufactured stones, Chart 171.

"Emerald, Synthetic" a MISNOMER ( COMMERCIAL LABEL ) for the green variety of synthetic corundum or spinel.

"Emerald, Uralian" a MISNOMER ( COMMERCIAL LABEL ) for demantoid garnet.

Emerald-cut name applied to a diamond cut in the trap (step) cut style, so called from the method of cutting emeralds. See Cuts of stones, Charts 181, 185.

"Emeraldine" a MISNOMER ( COMMERCIAL LABEL ) for a chalcedony stained green with chromic oxide. It is a deeper green than the stained chalcedony which owes its colour to nickel and, unlike the nickel type, shows a red residual colour under the dichromatic filter

"Emeraldite" a MISNOMER ( COMMERCIAL LABEL ) for green tourmaline.

"Emerita" the name given to the emerald coated beryls made by the Lechleitner process which causes a deposit of synthetic emerald on to the surfaces of previously fashioned aquamarine or yellow beryl. Now called "symerald".

Enantiomorphism crystals which show right and left-handed formation, not only in their outward appearance but in their optical properties also are said to exhibit enantiomorphism. These crystals show circular polarisation. Example, quartz.

Endoscope an instrument for the detection of cultured pearls. It depends upon the difference between the concentric structure of real pearls and the parallel structure of the motherof-pearl bead in cultured pearl. Can only be used for drilled pearls.

Enstatite R.I. 1-665 --- 1-674; S.G. 3-25 to 3-30; H. 5.5; Rhombic; Colour, green; Localities, South Africa. Burma.

Eosite an aventurine quartz with reddish veins. The best of this material is probably from the Altai Mountains of Russia.

Epiasterism a star effect seen in a suitably cut cabochon stone when light is reflected from suitably oriented inclusions within the stone. The asterism of star corundum is of this type.

Epidote R.I. 1-735-1-765 ; S.G. 3-25 to 3-50; H. 6 to 7; Monoclinic; Colours, yellow, green, pistachio green, brown, and red; Localities, Italy, France, Germany and Alaska. Alternative name "Pistacite".

Epithelium the name applied to the surface layer of cells of the "mantle" which covers the pearl oyster. These cells have secretory powers and produce the pearl-shell and the pearl.

"Erinide" an American trade-marked name for greenish synthetic spinel.

"Erinoid" a trade name for the formalised casein plastic.

Essence d 'orient a preparation of fish scales (from the underpart of the "bleak") which is used to produce the orient in imitation pearls.

Essonite see Garnet, Hesso''ite.

Etch figures small geometrical elevations or depressions produced on the faces of crystals due to solvent action, and which have a ciefinite relation to the internal crystal structure.

Ether (ethyl ether) a volatile inflammable liquid used as a test for amber and copal. Copal is softened by this liquid while amber is unaffected.

Ethylene dibromide a useful liquid with a low surface tension which thus may be used in place of water or density determinations by the direct weighing method. It is necessary to correct for temperature when using this liquid. See Correction Tables Chart 229.

Euclase R.I. 1-650 --- 1-669 to 1-652 --- 1-671; S.G. 3-05 to 3-10; H. 7.5; Monoclinic; Colours pale green, pale blue and colourless; Localities, Brazil and Russia.

Euhedral crystals crystals which show good faces and form.

"Evening Emerald" MISNOMER ( COMMERCIAL LABEL ) for peridot.

Extinction when a doubly refracting crystal is viewed in parallel polarised light with the Nicol prisms (or "Polaroid" discs) crossed, on rotation of the stage, the field becomes four times light and four times dark, i.e., extinction at 90 degrees. With isotropic materials the field remains dark at all positions. Isotropic material, when under strain, may show anomalous extinction but rarely orientated at 90 degrees. (Anomalous double refraction.)

Extraordinary ray that ray in a doubly refracting uniaxial mineral in which the velocity varies according to the direction in which it passes through the crystal.

Eyed agate agate in which the coloured bands are circularly arranged in such a way that they resemble an eye.

"Fabulite" trade name for strontium titanate.

Faces, Crystal see Crystalfaces.

Facets the name applied to the plane geometrical faces of a polished stone. The name is sometimes used also for the inclined faces at the top of a prismatic crystal.

Falcon' - eye" see Hawk's-eye.

False cleavage see Parting.

False lapis name sometimes applied to the stained chalcedony made to imitate lapis lazuli; also known as "Swiss lapis" or "German lapis".

"False topaz" see Quartz (citrine).

Fancy diamonds coloured diamonds which have a definite shade of colour, such as: canary yellow, green, blue, red, pink, brown, etc. Green and yellow coloured diamonds may have been treated. See also Coloured diamonds.

Fancy shapes the name used for oblong, pentagon, half-moon and other fancy shapes in cut-stones. See Chart 182.

Fashioning of stones A term used for the cutting and polishing of stones.

Fayalite one end member of the isomorphous series of which peridot is a variety. See Peridot.

Feathers layers of liquid or crystalline inclusions having a feather-like appearance sometimes found in natural crystals of rubies and sapphires, etc.

Feldspar (Felspar) name given to an important group of rockforming minerals, consisting of a silicate of aluminium with either potassium (Orthoclase and Microcline) or sodium and calcium (Plagioclase).

(1) Orthoclase; R.I. 1-52 --- 1-53 to 1-53 --- 1-54; S.G. 2-55 to 2-58; H. 6 to 6.5; Monoclinic; Colours yellow and colourless with opalescence (Moonstone); Localities, yellow, Madagascar; Moonstone, Ceylon, Switzerland and Burma.

(2) Microcline; R.I. 1-52 --- 1-53; S.G. 2-54 to 2-57; H. 6 to 6.5; Triclinic; Colour, bright verdigris green (Amazon-stone); Locality, Pikes Peak, Colorado (U.S.A.). The plagioclase group forms an isomorphous series from Albite to Anorthite. Only two members of this group come within the purview of gemmology, viz. Oligoclase (Sunstone or Aventurine) and Labradorite.

(3) Oligoclase (Sunstone or Aventurine); (70% to 90% of albite and 10% to 30% fanorthite);R.I. 1-5--- 1-55; S.G.262 to 2 65; H. 6 to 6.5; Triclinic; Colour, spangled bronze due to included flakes of haematite or goethite; Locality, Norway, Canada, etc.

(4) Labradorite; (30 % to 50 % of albite to 50 % to 70 % of anorthite); R.I. 1-56 --- 1-57; S.G. 2-70 to 2-72; H. 6 to 6.5; Triclinic; Colour, blue and grey with play of colour; Locality, Labrador, Canada.

F.G.A. (Fellow of the Gemmological Association) the suffix used by holders of the fellowship diploma of the Gemmological Association of Great Britain.

F.G.A.A. the suffix used by holders of the fellowship diploma of the Gemmological Association of Australia.

Fibrolite R.I. 1 658-1 677; S.G. 3 25; H. 7.5 Rhombic; Colours, pale blue and greenish; Localities, Burma and Ceylon. Alternative name "Sillimanite".

Figure stone a term used for material used for small stone carvings. It is generally agalmatolite. Fine light brown; a classification of gem diamond. Fine silver cape; a classification of gem diamond.

Fire the flashes of spectrum colours seen from the crown of a stone and due to dispersion, which see.

Fire Agate a layered dark brown color thats produces many and varried brilliant colors as light strikes the surface if cut or polished prorerly.

Fire-marble see Lumachella.

Fire marks small cracks seen along the facet edges in corundum. They are due to overheating on the polishing lap. They are seen both in natural corundums and in synthetic corundums, but are usually more prevalent in the latter.

Fire-opal see Opal.

"Fire pearl" a MISNOMER ( COMMERCIAL LABEL ) used for billitonite.

Firestones the name applied to rock crystal which has had cracks artificially produced by heat. See Quartz (iris quartz).

First bye a classification of gem diamond.

First cape a classification of gem diamond.

First water term used for diamonds which are perfectly limpid and free from flaws.

Flame-fusion process a process used in growing synthetic single crystals. A high temperature flame being used. The Verneuil process is a flame-fusion method.

Flats term used for those diamond crystals which are distorted octahedra.

Flaws may be fractures or cleavages either on the surface or running partly or completely through the stone. See Feathers.

Fleches d'amour see Quartz, Venus' Hair Stone.

Flint glass glass containing lead oxide which gives high dispersion. The material used for imitation gem stones usually has a range of R.I. from 1-58 to 1-68, and S.G. 3-15 to 4-15. The hardness is low in this type.

Fluorescence the effect exhibited by certain materials of producing visible light on being irradiated with invisible ultraviolet rays, cathode rays, X-rays or radium emanations. If the effect is continued after the removal of the exciting radiation it is then termed phosphorescence. See also Luminescence, and Chart 165.

Fluorite mineralogical name for fluorspar, which see.

Fluorspar R.I. 1-43; S.G. 3-17 to 3-19; H. 4; Cubic; Colours, violet, green, yellow, orange, blue, red, pink, brown and colourless; Locality, England, etc. Massive variety, Blue John (Derbyshire spar), Castleton, Derbyshire(England).

Flux fusion (Fluxed melt) a method of growing crystals,similar in many respects to crystallization from aqueous solutions, but the solvents used are solids with relatively high melting points. Some synthetic emeralds and rubies are grown by this method.

Foiled stones gemstones set with silver or coloured foil at the back in order to lighten a dark stone or to enhance the color 5 of a pale stone.

Form-birefringence the, usually small, birefringence commonly seen in material having a mineral of one refractive index embedded in another which has another index. Often seen in chalcedony.

Forsterite one end member of the isomorphous series of which peridot is a variety. See Peridot.

Fossil ivory ivory obtained from the tusks of extinct mammoths whose bodies have been preserved in solid ice since Ice Age times. Found mainly in Siberia and North America. Some of this material is suitable for use in the arts. See Mammoth ivory.

Fossil resin see Amber.

"Fossil turquoise" a MISNOMER ( COMMERCIAL LABEL ) for odontolite. See Odontolite.

Four-point see Diamond point.

Fracture the term used to describe the surface of a stone when chipped or broken. The character of the surface varies with different stones and names are applied to the different kinds: Conchoidal; when the surface takes the form of many more or less concentric ridges resembling the lines on certain shells. Even; when the surfaces are smooth and even. Uneven; when the surfaces are rougher, this is sometimesternied "hackly fracture". Splintery; when the substance breaks into long splinters. Fibrous substances produce this type.

Fraunhofer lines dark lines seen in the spectrum of the sun, which represent the absorption of certain wave-lengths of light by elements present in the outer-chromosphere. Certain of these lines are denoted by alphabetical letters and may be used for calibration. See Chart 155.

Freshwater pearls pearls found in the pearl mussels which inhabit inland waterways of Europe and America. See Mussel pearls.

Frictional electricity charge of electricity developed by certain substances vigorously rubbed with a cloth; diamond, tourmaline and topaz taking on a positive charge and amber a negative one. (Some plastic imitation embers also show this effect.)

Friedelite R.I. 1 63-1 66; S.G. 3 07; H. 4 to 5; Trigonal; Rose red to Orange-red; Localities, France, Sweden and the U.S.A.

Frosted Diamonds diamond crystals which have a frosted appearance but are not so heavily coated as "coated" crystals. See Coated diamonds.

Fusibility the relative fusibility of minerals, generally determined from a scale of six minerals, suggested by Von Kobell, in which the temperature of fusion is understood to increase by approximately equal amounts. Von Kobell's scale is as follows:

1. Stibnit.................................... melting at 525 C.

2. Natrolite ................................................. 965 C

3. Almandine garnet.................melting at 1,200 C

4. Actinolite...............................................1,296 C

5. Orthoclase..............................................1,300 C

6. Bronzite.................................................1,380 C

Gahnite R.I. 1-805; S.G. 4-40; H. 7.7 to 8; Cubic; Dark green; many localities.

Gahnospinel blue spinel containing considerable proportion of zinc. Found in Ceylon. S.G. up to 4-06; R.I. up to 1-753.

Garnet an isomorphous series of minerals (gem-stones) represented by the formula R"3R"'2, where R" may be any of the bivalent metals magnesium, calcium, manganese or iron, while R"' stands for a trivalent metal, aluminium, ferric iron or chromium. All garnets crystallise in the cubic system. The garnets are divided into two series; the "pyralspite series" which consists of pyrope, almandine and spessartite; and the "ugrandite group" consisting of uvarovite, grossular and andradite.

(1) Grossular; R.I. 1 742 to 1 748; S.G. 3 55 to 3 67 ("Transvaal Jade", 3-42 to 3-72; H. 6.5 to 7;

(A) ) Hessonite Colours, brownish yellow (Cinnamon-stone) reddish orange, transparent green massive green ("Transvaal Jade"), and other colours; Localities, Ceylon, Switzerland and South Africa.

(2) Pyrope; R.I. 1-74 to 1-75; S.G. 3-68 to 3-80; H. 7.5; Colour, red; Localities, South Africa, Bohemia, Arizona, etc.

(3) Spessartite; R.I. 1-79 to 1-81; S.G. 3-90 to 4-20; H. 7.5; Colours, brown, orange-red and hyacinthred; Localities, Bavaria, Ceylon, Tyrol and U.S.A.

(4) Almandine; R.I. 1-75 to 1-82; S.G. 3-80 to 4-20; H. 7.5; Colours, deep red, violet-red and black; Localities, Australia, India, Ceylon and North and South America.

(5) Four-rayed asterism sometimes seen.

(6) The lower range given here for the almandine is sometimes described as the pyrope-almandine series.

(7) Uvarovite; R.I. 1-84 to 1-85; S.G. 3-41 to 3-52; H. 7.5; Colour, bright green; Locality, Russia.

(8) Andradite; R.I. 1-82 to 1-89; S.G. 3-80 to 3-90; H. 6.5; Colours, green (Demantoid, R.I. 1-88 to 1-89; S.G. 3-83 to 3-85), yellow (Topazolite) and black (Melanite); Localities, Russia, Saxony and Italy.

(9) Rhodolite; a violet variety in composition lies between pyrope and a]mandine in the ratio of 2:1. R.I. 1-76; S.G. 3-84; H. 7.25; Locality, Macon County, North Carolina (U.S.A.).

Garnet-topped doublets composite stones made with a garnet top on a glass base.

Garnet-type synthetic stones crystals which have been grown synthetically. The most important as gemstones are the yttrium aluminium garnets (Diamonair).

Gauge, Diamond see Diamond gauge.

"Garnet, Synthetic" a MISNOMER ( COMMERCIAL LABEL ) for the synthetic dark red corundum made to represent the garnet colour.

Gel a jelly, a colloid.

Gem-cut diamond any diamond cut as a gem, it is a United States definition for export purposes.

Gemmology the science which deals with precious minerals and other materials used for personal adornment, and for objets d'art.

Gemolite an American instrument used for observing imperfections and internal features in diamonds and coloured gemstones. The instrument functions similarly to the Diamondscope, which see.

Gemological Institute of America a school that is run by the jewelry industry for training in jewelry related fields. Courses are offered by correspondance and at several fixed campuses.

Gemolux a Swiss instrument incorporating a lighting device for the examination of stones.

Gem peg see Jamb peg.

Gems, Artificial see Artificial gems.

Gems, Imitation see Imitation gems.

Gem-stick the appliance used by the lapidary for holding the rough stone when cutting the facets. Of a penholder-like form with a metal collet at the end filled with cement into which the material to be cut is fixed, leaving exposed just sufficient to cut either the front or the back facets. See Jamb peg.

Geodes crystal-lined cavities found in rocks.

Geology the science which treats of the structure and mineral constitution of the earth.

German lapis jasper or chert stained blue and used to imitate lapis-lazuli.

Ghost crystals the so-called "phantom" or "ghost" crystals of quartz appear to contain one or more crystals similar to it. The effect is due to interruption of growth.

GIA abreviation for the Gemological Institute of America.

Gilson synthetic emerald a synthetic emerald produced by the firm of Pierre Gilson, of France.

Girasol see Opal.

Girdle name applied to the outer edge of a cut stone. It is the line of junction of the top (crown) and the base (pavilion). Also termed the setting edge.

Glycerine a sweet colourless viscid liquid obtained from oils, fat, etc. The refractive index is 1-47. See Chart 131.

Gneiss a banded metamorphic rock derived either from igneous rocks (Ortho-gneiss) or sedimentary-rocks (Paragneiss).

Goethite Hydrated iron oxide, Fe203H2O, is often an inclusion in oligoclase feldspar giving rise to the variety sunstone.

Gold the precious metal of yellow colour used as a setting for precious gems. See also Carat and Tests for precious metals, Chart 219. See also Yellow gold, Red gold, Creen gold, Blue gold, etc.

"Gold Buss" another name for Goldstone.

Gold opal an alternative name for fire opal.

Gold quartz milky quartz containing particles of native gold. This is sometimes cut and polished for jewelry.

Goldstone a paste (glass) imitation of reddish-brown aventurine made by the inclusion of copper crystals in the glass. See Aventurine glass.

Goniometer an instrument for the measurement of crystal angles.

Goshenite colourless beryl, which see.

Grain a unit of weight (Troy and Avoirdupois) see Chart 224; the pearl grain is one fourth of a carat, see Chart 224.

Gramme (Gram) unit of weight of the metric system. Some ornamental stones are sold by the gramme.

Granite A granular igneous rock containing principally quartz, feldspar and mica. It is sometimes cut and polished for small jewelry articles.

"Green garnet" a name at one time incorrectly used for enstatite.

Green gold gold in which a greenish colour is induced by considerably increasing the proportion of silver in the 14 ct. quality. Several shades are obtained by careful manipulation of the percentages. Used as a contrast.

Green rouge a polishing powder consisting of chromium oxide.

Greenstone see Nephrite.

"Green stone" a MISNOMER ( COMMERCIAL LABEL ) sometimes used for chlorastrolite. See Chlorastrolite.

Grey gold a grey colour induced in a gold alloy by the addition of iron or steel. It has a limited application.

Grit an alternative name for diamond powder.

Grossular see Garnet.

Gypsum R.I. 1-52 --- 1-53; S.G. 2-2 to 2-4; H. 2; Monoclinic; Colour, white (Satin spar and Alabaster); Localities, England, U.S.A. and Italy. See also Selenite.

Habit the characteristic crystal form displayed by a mineral. Habit may vary with locality.

Haematite R.I. 2-9 --- 3-22; S.G. 4-9 to 5-3; H. 5.5 to 6.5; Trigonal; Colour, black; Localities, England, Germany, Scandinavia and U.S.A. Sometimes used to imitate the black pearl. The name hematite should not be used for the heliotrope variety of chalcedony.

Half-facets see break facets.

Hambergite R.I. 1-55--- 1-62; S.G. 2 to 3-5; Rhombic; Colourless; Locality, Madagascar.

Hardness the relative ability of one type of stone to wear on another type of stone. Scale of 10 as hardest to 1 as softest.

Hardness (H) points or pencils small conical fragments of minerals set in a pencil-shaped holder for ease in testing the hardness of a stone.

Diamond (10)

Sapphire (9)

Topaz (8)

Quartz (7)

Feldspar (6)

Harlequin opal; see Opal.

Hauynite a blue minera1 of cubic symmetry, it is a sodium aluminium silicate and is one of the constituents of Lapislazuli; also known as Hanyne.

Hawk's-eye quartz pseudomorphous after crocidolite (blue asbestos) which has not suffered oxidation. See also Crocidolite.

Heat treatment the controlled heating of certain stones, such as zircon and topaz, in order to effect a change of colour. See Chart 178.

Heavy liquids liquids having a high density and suitable for the determination of the specific gravity of gem-stones. See Chart 118.

Heliodor yellow beryl.

Heliotrope see Chalcedony.

Hematite see Ha'matite.

Hemihedral forms those crystals which show only half the number of faces required to conform to the symmetry of the normal crystal.

Hemimorphism the peculiarity possessed by certain crystals, presenting different forms at opposite ends of an axis of symmetry.

Herderite R.I. 1 59-1 62; S.G. 3 0; H. 5; Greyish; Monoclinic; Saxony and U.S.A.

Hessonite see Garnet, Grossular.

Hexagonal one of the crystal systems. see Chart 111.

Hexagonite see Tremolite.

Hiddenite see Spodumene.

Hippopotamus ivory an ivory obtained from the canine teeth of the hippopotamus. (Hippopotamus amphibus).

Holohedral forms those crystals which show the full number of faces for the full symmetry of the system.

Homogeneous the term used for material which is composed of similar parts of elements. It is uniform throughout.

"Honan jade" an Oriental name variously applied to a jade and quartz mixture, or to serpentine or soapstone.

"Hope sapphire" a MISNOMER ( COMMERCIAL LABEL ) that was suggested for the synthetic blue spinels that were the outcome of experiments in the production of synthetic blue corundum. The use of magnesia as a flux to cause even distribution of the blue colour gave the unexpected result that spinel was produced and not corundum.

Hornblende a rock-forming mineral similar in composition to nephrite, may be black or green (Smaragdite).

"Hot Springs diamond" a MISNOMER ( COMMERCIAL LABEL ) for rock crystal.

Howlite an opaque white ornamental stone veined with black consisting of an aggregate of monoclinic (?) crystals; R.I. 1-59; S.G. 2-58; H. 3.5; has been stained a turquoise-blue colour; California.

"Hungarian cat'seyes" an unsatisfactory name for quartz cat's-eyes from Bavaria.

Hungarian opal the name applied to opal from the Cervenica mines, Hungary.

Hyacinth name applied to the orange-red zircon or to the similarly coloured hessonite garnet. An obsolete name.

Hyalite see Opal.

Hydrochloric Acid a powerful mineral acid formed by the solution of the gas HCI in water. One of the constituents of aqua regia.

Hydrogrossular correct name for the massive grossular garnet, which see.

Hydrometer an instrument for determining the density of liquids.

Hydrophane see Opal.

Hydrostatic weighing the direct weighing method for determining specific gravity. See Chart 117.

Hydrothermal method a method of growing crystals by the use of a saturated water solution at a pressure of 1,000 atmospheres and temperatures of less than 400 C. An autoclave is used in this process.

Hypersthene R.I. 1-67-1-68 to 1-69 --- 1-70; S.G. 3-3 to 3-4; H. 5 to 6; Rhombic; Colour, dark with metalloidal iridescence. Hypersthene is related to enstatite an intermediate form being bronzite.

"Iceland agate" a MISNOMER ( COMMERCIAL LABEL ) for brownish or greyish obsidian.

Iceland-spar see Calcite.

Ideal cut the name given to a diamond cut to the correct Proportions for maximum brilliance.

Idiochromatic minerals those in which the colouring is due to some essential constituent, the colour being constant and therefore of assistance in identification, e.g., Malachite. See also Allochromatic minerals.

Idocrase R.I. 1-702 ---1-706 to 1-72 --- 1-732; S.G. 3-35 to 3-45; H. 6.5; Tetragonal; Colours, yellow, green, brown, blue (containing copper and called Cyprine), compact green (Californite); Localities, Italy, Siberia, Norway and U.S.A. Alternative name, Vesuvianite.

Igneous rocks solidified molten magma of mineral matter, mostly of complex composition.

"Igmerald" trade name given to the synthetic emerald produced in Germany; a true synthetic beryl having constants and colour near those of natural emerald. May be detected by certain characteristic internal markings. Not now made.

Illam name given to the gem-bearing gravel of Ceylon.

Imitation stones a man-made stone that is made to resemble a genuine stone but that is not chemically or structurally the same.

Immersion liquids a series of liquids that are of different specific gravities so that a stone can float or sink to determine if it is heavier or lighter than the liquid.

Imperial jade the name given to the apple-green or emeraldgreen translucent jadeite. A highly prized colour.

"Imperial mexican jade" a MISNOMER ( COMMERCIAL LABEL ) for green-dyed calcite. Inanga; Maori name for the grey variety of nephrite.

Inca rose a fancy name for Rhodochrosite.

Inca stone iron pyrites.

Inclusions general name for crystals and liquid- or gas-filled cavities enclosed in a "host" mineral. Such inclusions are often highly characteristic of the mineral concerned and are the surest basis for the distinction between natural and synthetic stones.

Index of refraction The relative amount that light is bent as it enters a gem. Used as a way of identifing gems.

"Indian emerald" a MISNOMER ( COMMERCIAL LABEL ) for green-dyed crackled quartz (rock crystal).

"Indian jade" aventurine quartz, which see.

"Indian topaz" a MISNOMER ( COMMERCIAL LABEL ) for yellow corundum.

Indicators pieces of glass or small fragments of gem-stones of known specific gravity used to indicate the approximate density of heavy liquids.

Indicolite see Tourmaline.

Industrial diamonds diamonds used for industrial purposes, such as deep well boring, turning tools for metals and plastics, truing abrasive wheels, dies for fine wiredrawing, hardness indenters, and as powder for grinding and polishing metals.

Infra-red (Heat rays) electro-magnetic radiations of wavelengths between 7,900 Angstrom units and 10,000,000 Angstrom units, that is beyond the visible red. These radiations produce in certain minerals a luminescence. See Thermo-luminescence.

Inorganic not produced by vital processes.

Interfacial angle the angle between any two faces of a crystal. In crystallography, the interfacial angle is the angle between the normals, or perpendiculars, to the two faces.

Interference figures the figures seen when anisotropic minerals are viewed in convergent polarised light. They afford useful information concerning the optical characters.

Interference of light where two rays travelling in the same path, but out of phase, mutually interfere with one another causing either total extinction of light or predominance of one or more colours. This is the cause of the play of colour in opal and thin films.

Internal strain local straining of the atomic structure due to pressure caused by solid inclusions within the crystal, or through rapid cooling, as in some glass and synthetic gems. This strain gives rise to anomalous double refraction.

Interpenetrant twins where two or more crystals penetrate each other along a common twinning direction, common in fluorspar.

Iodobenzene a liquid having a value in certain refractive tests. R.I.= 1 62.

Iodo-naphthalene (a type) a liquid having a refractive index of 1704 which is suitable for use as a contact liquid for the Rayner spinel type refractometer.

Iolite R.I. 1-53 --- 1-54 to 1- 5 --- 1-55; S.G. 2-58 to 2-66; H. 7 to 7.5; Rhombic; Colour, blue (strongly pleochroic); Locality, Ceylon; Alternative names, Cordierite, Dichroite.

Ion an electrically charged atom, radicle or molecule.

Iridescence the prismatic colours seen in cracks and flaws in a stone, well seen in iris quartz. Due to interference of light at thin films of differing refractive index. The surface tarnish in haematite, etc., is due to this effect.

Iridium Ir. S.G. 22 41; one of the platinum group of metals having little application in jewelry except in the hardening of gold and platinum.

Iris rainbow quartz; see Quartz.

Irish diamond a MISNOMER ( COMMERCIAL LABEL ) for rock crystal.

Irradiated diamonds diamonds which are artificially coloured by bombardment with atomic particles or gamma rays. See also Cyclotroned diamonds, Radium-treated diamonds, Pile-treated diamonds and Electroned diamonds. Also called Atomic Diamonds.

Irregulars the term used for gem diamond crystals of irregular shape, or long shape.

"Isle of Wight diamonds" a MISNOMER ( COMMERCIAL LABEL ) for rock crystal.

Isomorphous replacement where one element in the chemical composition of a mineral is replaced by another element having the same valency without sensibly altering the structure and form of the crystal. This causes wide variations in the physical properties, e.g., Garnet.

Isotropic the term used to denote all materials which are singly refractive to light, that is, light travels through them as one ray and having the same velocity and character in all directions. All crystals of the cubic system and all amorphous materials are isotropic. See also Anisotropic.

Ivory (dentine) an organic substance from the tusk of the elephant, walrus, hippopotamus, narwhal, etc. S.G. 1-70 to 1-98; R.I. 1-54; H. 2.5.

Ivory (vegetable) the hard white kernel of the nut of certain palm trees (in particular that of Phytelephas Macrocarpa from South America) ; S.G. 1-38 to 1-42 ; R.I. 1-54 ; H. 2.5.

Jacinth an unsatisfactory name variously applied to the red zircon and to the similarly coloured hessonite garnet.

Jade a term used for both jadeite and nephrite (which see). Other massive material sometimes confused with the jades are as follows:

(1) Californite; green compact idocrase. Grossular Garnet; Pseudophite;

(2) Serpentine

(a)Antigorite (thin and platy).

(b)Bowenite (c)Williamsite

(3) Hornblende;

(4) Quartz; Chalcedony (Chrysoprase and Plasma).

(5) Saussurite,

(6) Smithsonite;

(7) PreLnite,

Jadeite ("Jade" part) R.I. 1-65 to 1-68; S.G. 3-3 to 3-5; H. 6.5 to 7; Monoclinic; Colours, white, green, pink, lilac, mauve and brown; Locality, Burma. See also Chloromelanite.

Jade matrix (Jade albite) a rock consisting of a mixture of nephrite and albite. Sometimes called "snowflake jade".

Jam-Peg an upright post, positioned at the side of the polishing lap and containing a number of suitably placed holes into which is placed the end of the gem holder, thus regulating the angle of the facet being cut. See Gem stick.

"Japanese pearls" cultured blister pearls finished with a flat base.

"Japan pearls" cultured blister pearls finished with a spherical base. Sometimes called Half-cultured pearls.

Jargoon outmoded name for pale coloured zircons.

Jasper an impure form of compact quartz.

Jet a form of fossil wood allied to cannel coal; R.I. 1-64 to 1-68; S.G. 1-10 to 1-40; H. 3.5; Localities, Spain, and Yorkshire (England).

Jolly's spring balance a balance consisting of a spring suspended vertically against a graduated scale. At the lower end of the spring is attached two scale pans one above the other, the lower pan being immersed in water. The density is determined by recording the reading on the scale of a pointer situated at the lower end of the spring when there is no specimen in either of the pans (reading A), when the specimen is in the top pan, that is in air (reading B), and when specimen is in the lower pan, that is in water (reading C). Formula: B minus A divided by B minus C gives the specific gravity.

"Jourado diamond" a name, reported from the U.S.A., as being an incorrect term for an imitation colourless stone. The term may have had its origin from the so-called "diamond scare" of 1935 in England, in which case the material involved was synthetic colourless spinel. The term is definitely a MISNOMER ( COMMERCIAL LABEL ).

Kahurangi Maori name for the pale green and translucent variety of nephrite.

"Kandy spinel" a MISNOMER ( COMMERCIAL LABEL ) for the reddish-violet garnet found in Ceylon.

Kaolin a water-containing silicate of alumina, a product of the disintegration of different rocks. The levigated material is used as a polishing agent.

Karet mesurement of Diamonds; can be found in all shapes and sizes. The size of the diamond is determined by it's weight in carats. A carat is divided into points is a .75 carats or 3/4 of a carat. Larger diamonds are more rare, and command a higher price. While size is an obvious factor in determining the value of a diamond, bigger is not necessarily better, all 4 C's should be considered.

Kauri gum see Copal resin.

Kawakawa Maori name for the ordinary green variety of nephrite.

Keystone the name applied to a trap-cut stone having an outline in the form of the conventional keystone. See Cuts of stones, Chart 182.

Kidney stone a name variously applied to either nephrite or to haematite.

"Killiecrankie diamond" a MISNOMER ( COMMERCIAL LABEL ) for colourless topaz from Tasmania.

Kimberlite a variety of peridotite, a basic igneous rock which fills the diamond pipes of South Africa. Known also as "Blue ground" which weathers by oxidation at the surface to "Yellow ground".

"King topaz" a MISNOMER ( COMMERCIAL LABEL ) for yellow corundum.

Kite the name applied to a trap-cut stone having an outline in the shape of a kite. See Cuts of stones, Chart 182.

Kites name sometimes applied to the eight crown facets known as the bezels and quoins.

Knot term used for an irregularity of crystal structure in a single diamond crystal. Owing to the difficulty of finding the cleavage planes such rough diamond is not easily fashioned. Also known as Naat.

"Korea jade" a MISNOMER ( COMMERCIAL LABEL ) for green serpentine.

Kornerupine near R.I. 1-665 --- 1-678; S.G. 327 to 3-32; H. 6.5; Rhombic; Colours, yellow, brown and green; Localities, Madagascar, Ceylon, Saxony and Greenland.

Kunzite violet-pink spodumene, which see.

Kyanite R.I. 1 712-1 728; S.G. 3 55 to 3 67; H. 5 to 7 and varies with direction; Triclinic; Colours, colourless, sky-blue, green and brown; Localities, Switzerland, Kenya, Brazil and U.S.A. Alternative spelling Cyanite.

Labradorescence name given to the play of colour seen in the labradorite feldspar.

Labradorite a variety of plagioclase feldspar showing a play of colour (labradorescence). See Feldspar.

Labrador moonstone labradorite.

Labrador spar labradorite.

"Lake George diamond a MISNOMER ( COMMERCIAL LABEL ) for rock crystal.

Lake Superior greenstone chlorastrolite.

Lamellar structure when the mineral is composed of thin layers of scales like the leaves of a book the structure is said to be lamellar.

Landerite see Xalostocite.

Lap the name applied to the disc upon which gemstones are cut and polished. For diamond the lap is of hard cast iron. It is also known as scaife. With the softer gemstones the grinding laps are made of copper, gunmetal or lead, while pewter or wooden laps, sometimes faced with cloth or leather are used for polishing.

Lapidary the name applied to craftsmen who cut and polish gemstones other than diamonds. The craftsmen who fashion diamonds go under the name of "Diamond cutters" and not lapidaries.

Lapis, False see False lapis.

Lapis-lazuli (Lazurite) a mixture of several minerals, chief of which are lazurite, and calcite; R.I. 1-50; S.G. 2-75 to 2-90; H. 5.5; Colour, deep blue often with spangles of pyrites; Localities, Afghanistan, Chile, Siberia and California.

Lapper the name applied to the diamond cutter who cuts the main facets of a brilliant cut stone.

Lasque a flat thin diamond used by Indian jewellers. May be the origin of the term "laxy diamond" used in the trade for rather flat diamonds cut in the brilliant style.

Lauegram the characteristic figure of spots produced on a photographic film when a narrow beam of X-rays is passed through a single crystal, due to diffraction from the atomic layers. The six-spot and four-spot X-ray photographs of pearls and cultured pearls are a modification of this effect.

"Lavernite" a name applied to the synthetically produced periclase.

"Laxy diamonds" Slang trade term for rather flat brilliant cut diamonds. See Lasque.

Lazulite R.I. 163; S.G. 31; H. 5 to 6; Monoclinic; Colour, blue; Localities, Austria and U.S.A.

Lazurite see Lapis-lazuli.

Lead glass flint glass containing a high percentage of lead oxide. This increases the dispersion but decreases the hardness. See Flint glass.

Lechosos opal see Opal.

Lepidolite R.I. mean 1-55; S.G. 2-8 to 2-9; H. 3.5; Monoclinic; A rose red ornamental stone from Russia, Madagascar and U.S.A.

Leucite KAl(Si3O2)2; R.I. 1 51; S.G. 2 48; H. 51 to 6; Cubic; Colourless; from Italy.

Leuco-sapphire colourless corundum, which see.

Leveridge gauge A stone gauge on the principle of calipers with the measurements shown on an indicating dial.

Light 1 a form of radiant energy, travelling through space at the speed of 186,285 miles per second, which gives rise to the sensation of sight. It is an "octave" of the electromagnetic spectrum and differs from wireless, heat, ultraviolet and X-rays solely in wave-length.

2 Interference of; see Interference of light.

3 Monochromatic; see Monochromatic light.

4 Reflection of; see Reflection of light.

5 Refraction of; see Refraction of light.

6 Speed of; see Speed of light.

Lime name commonly applied to calcium oxide, also used for a polishing product consisting of unhydrated Dolomite containing a high percentage of magnesia. It may be said to be a double oxide of calcium and magnesium and has almost entirely replaced crocus.

Limonite a hydrated iron oxide which is often the matrix in which is found turquoise. Brown in colour, limonite is sometimes cut with turquoise to produce what is known as turquoise matrix.

Linde emerald a synthetic emerald made by the hydrothermal method by the American firm of Linde.

"Linobate" lithium niobate, a synthetically produced crystal used as a gemstone.

Lintonite a plain green variety of thomsonite.

"Lithia amethyst (Lithium Amethyst)" a MISNOMER ( COMMERCIAL LABEL ) for the kunzite variety of Spodumene, which see.

"Lithia emerald" a MISNOMER ( COMMERCIAL LABEL ) for the green hiddenite variety of spodumene, which see.

Lithium niobate (LiNbO31; a synthetically produced crystal used as a gemstone under the name 'linobate'. The hardness is 6; the density 4 64 and the refractive indices 2 21-2 30.

Liver opal see Opal.

Logarithmic tables Calculation tables.

Loupe jewelers see Lens.

Low-type zircon the division of zircon consisting of those stones, generally green in colour, which have low physical and optical constants and are practically amorphous. This type, which is said to be the result of a more or less complete breakdown of the crystalline types, shows practically no birefringence.

Lozenge the name applied to a trap-cut stone having a lozenge-shaped outline.

Lucidoscope the name applied to an early method of cultured pearl testing. The pearl is immersed in monobromonaphthalene, illuminated by a strong source of light and viewed microscopically. On being slowly turned a natural pearl will show an equal density to the light, whereas a cultured pearl may show a banded structure when the mother of pearl layers of the bead nucleus are in the same plane as the light rays.

Lucinite variscite from Lucin, Utah.

Lumachella or Lumachelle "Fire Marble", a marble containing fossil shells which show a play of colour reminiscent of opal.

Luminescence the effect noticed in some substances of giving out visible light when they are rubbed or scratched (Triboluminescence) or when they are irradiated with invisible electro-magnetic radiations (Fluorescence, Phosphorescence and Thermoluminescence).

Lunette the name applied to a trap-cut stone with an outline having one side a straight line and the other a segment of a circle. See Cuts of stones, Chart 182.

Lustre the brilliancy of a stone by reflected light, determined by the amount of incident liaht reflected from its surface. Types of lustre as follows:

(1) Adamantine; as in diamond and some zircons. Possessed only by minerals of high refractive index.

(2) Vitreous; glass-like as in quartz and most gemstones.

(3) Resinous; as in certain garnets.

(4) Waxy; as in turquoise.

(5) Pearly; as in moonstone.

(6) Silky; as in satin-spar.

(7) Metallic; as in gold.

The lustre of minerals may be of different degrees or intensity and are then described as under: Splendent; the surface reflects light as in a mirror Shining; objects are reflected indistinctly. Glistening or Glimmering; denotes a still more feeble lustre. Dull; little or no lustre.

"Lux sapphire" a MISNOMER ( COMMERCIAL LABEL ) for Iolite.

"Lynx sapphire" a MISNOMER ( COMMERCIAL LABEL ) for the pale blue shade of Iolite, which see.

Mabe pearls cultured blister pearls in which the original nucleus has been removed and replaced by a smaller bead cemented in with white wax and the pearl completed with a dome-shaped backing piece of mother-of-pearl.

Macles flat, triangular twin crystals of the octahedron. Term used for diamond crystals of this type.

Madagascar aquamarine a blue beryl from Madagascar, showing strong dichroism.

"Madeira topaz" a MISNOMER ( COMMERCIAL LABEL ) for citrine.

Magma deep seated molten material from which igneous rocks form.

Magnesite R.I. 1 52-1 72; S.G. 3 0 to 3 12; H. 4; Trigonal; White; World wide, but Brazil is important.

Magnetic spectrum see Electro-magnetic spectrum.

Malachite R.I. 1 65-1 90; S.G. 3 74 to 395; H. 32; Monoclinic; Colour, green; Localities, Ural Mountains (Russia), Chile, Rhodesia, U.S.A., Katanga (Belgian Congo).

Malacolite see Diopside.

Mammoth ivory fossil ivory from the tusks of the mammoth (Elephas primigenius), found in Siberia. And from allied animals in North America. See Fossil ivory.

Manganese dioxide a black powder sometimes used as a polishing agent.

Mantle the cellular epithelial tissue of the pearl producing molluscs; that part of the animal which secretes the nacre.

Manufactured stones synthetic stones, see Chart 167.

Maori stone name for nephrite from New Zealand.

Marble a massive form of crystalline limestone. See Calcite.

Marcasite S.G. 4-8; H. 6 to 6.5; Rhombic; Colour, metallic grey yellow (this is a mineral dimorphous with pyrites); "Marcasite" in the trade is really pyrites, or often merely cut steel or white metal.

"Marmora diamond" a MISNOMER ( COMMERCIAL LABEL ) for rock crystal.

Marquise a stone cut on the brilliant style with the exception that the outline is boat-shaped instead of having a circular form.

"Mass aqua" the popular name for a special hard glass imitation of aquamarine.

Massive a term used for minerals without definite crystal form, or composed of masses of small crystals.

Matrix the mass of rock in which minerals and crystals are found. Turquoise and Opal are often cut with some of the matrix. Also called Mother rock.

"Mature diamond" a MISNOMER ( COMMERCIAL LABEL ) for the colourless (fired) zircon.

Maw-Sit-Sit the name applied to a jadeite-albite rock found in Upper Burma.

Maxixe-aquamarine, Maxixe-beryl a blue beryl showing strong dichroism, from the Maxixe mine, Minas Gerais, Brazil.

"Medina emerald" a MISNOMER ( COMMERCIAL LABEL ) for emerald green glass.

Medium the term applied to any material, solid, liquid or gaseous, through which light passes. The medium may not necessarily be transparent for even in opaque substances light may penetrate to a very small depth.

Meerschaum R.I. mean 1-55; S.G. 1-0 to 2-0 H. 2 to 2.5; Monoclinic; Colour, creamy white; Locality, Asia Minor. Also known as Sepiolite.

Melange term used for diamonds of mixed sizes but larger than .25 carat in weight.

Melanite a black variety of andradite. See Garnet.

Melee small diamonds of mixed sizes, less than .25 carat in weight.

Melinophane R.I. 1-59 --- 1-61; S.G. 3-0; H. 5; Tetragonal; Yellow; Norway.

Menilite alternative name for liver opal. See opal.

Mercury-vapour lamp an electric discharge lamp producing an intense bluish light rich in ultra-violet rays. It is operated by the arcing of mercury vapour in a vacuum tube which is generally made of quartz glass.

Mesolite a sodium calcium aluminium silicate ; R. l. 150 ; S.G. 2 29; H. 5; Monoclinic; White or colourless (fibrous) World wide occurrences.

meta-made an artificial stone made in Japan. It resembles, and has properties near to, jade.

Metamict the term used for material which has suffered a break-down from a crystalline to an amorphous state; common in minerals containing radio-active elements, it is the name applied to the "low" type of zircon which has decomposed into nearly amorphous SiO2 and ZrO2 from the "normal" fully crystalline zirconium silicate. Stones of this type, mostly found in Ceylon, are nearly singly refracting and generally green in colour. Prolonged heating returns these stones to the "normal" type.

Metamorphic rocks are those formed from igneous or sedimentary rocks by the action of heat and pressure.

Methylene iodide a liquid used for immersion purposes in the microscopical examination of gemstones, for certain refractive index tests and as a heavy liquid. R.I. 1 74; S.G. 3 32.

Methylene iodide and dissolved sulphur a mixture used as a refractometer contact liquid and as an immersion liquid in certain refractive index tests. R.I.= 1 78.

Methylene Iodine, dissolved sulphur and tetraiodoethylene a highly refractive liquid used as a refractometer contact fluid and as an immersion liquid in certain refractive index tests. R.I. = 1 81.

Methylene iodide with dissolved iodine and iodoform a highly refractive liquid used as a refractometer contact fluid and as an immersion liquid in certain refractive index tests. R.I. depends on mixture

Metric carat the legal weight unit for gemstones, one-fifth of a gramme. See Chart 224.

"Mexican diamond" a MISNOMER ( COMMERCIAL LABEL ) for rock crystal.

"Mexican jade" a MISNOMER ( COMMERCIAL LABEL ) for green-dyed calcite.

"Mexican onyx" a MISNOMER ( COMMERCIAL LABEL ) for banded calcite.

Mexican water opal a variety of Opal which shows a play of colour in a practically transparent body.

Nlica schist a general term for a laminated rock in which mica is an abundant ingredient.

Micro chemical tests tests carried out by the observation of chemical reactions on minute portions of a mineral. A scraping of the substance to be examined being placed on a glass slip and the reactions to various reagents studied under the microscope.

Microcline see Feldspar.

Microlite 1-93; S.G. 5-5; H. 5.5; Cubic; Brownish-green, yellowish-brown to hyacinth-red; Elba and U.S.A.

Micron a unit of measurement for small sizes. It is 0 001 mm. and is used in the measurement of the grains in diamond powder.

Microscope an instrument consisting of a system of lenses fitted into a tube which can be raised or lowered for focusing, which is used to give a greatly enlarged image of an object. A petrological microscope contain$ in addition, two nicol prisms, in order that observations may be made in polarised light.

Milk-opal see Opal.

Milky quartz crystalline quartz of a milky white colour When containing gold, is termed "gold quartz".

Mineralogy the science which deals with minerals.

Mineral species a homogeneous substance produced by the processes of inorganic nature, having chemical composition, crystal structure, and physical properties which are constant within narrow limits.

Mixed-cut a style of fashioning gem stones. Used to denote an unusual cut.

Minimum deviation the position of two facets forming a prism, in relation to a beam of light, where the beam is passing through them symmetrically, or in other words, with least deviation. (For Refractive index determination by the method of minimum deviation, see Chart 129.)

Mocha stone chalcedony with dendritic inclusions.

Moe's gauge a diamond gauge on the principle of calipers, which by measuring across the diameter of the stone and also its depth and referring to tables it is possible to estimate the weight of the stone.

Mohs' scale a scale of the hardness (H) of minerals. 10 is the hardest with 1 being the softest.

"Moiave moonstone" a MISNOMER ( COMMERCIAL LABEL ) for lilac-tinted chalcedony from the Mojave Desert, California.

Moldavite a silica glass found in Bohemia and Moravia and not unlike obsidian. R.I. 1-48 to 1-50; S.G. 2-3 to 2-5; H. 5.5; Amorphous; Colour, light to dark green, rarely brown; Alternative names, "Water chrysolite" and "Bottle-stone".

Molecular weight of a substance is the sum of the atomic weights of the atoms composing a molecule oi the substance, e.g., the atomic weight of calcium is 40, of carbon 12, and of oxygen 16; therefore the molecular weight of calcite , composed of one atom of calcium, one of carbon and three of oxygen, is 40+12+(3 x 16) = 100.

Molecule a group of two or more atoms in close combination. The smallest quantity of an element or compound which can exist alone.

Mollusc a member of the sub-kingdom of soft bodied, and usually hard-shelled, animals, as the oyster, scallop, mussel, etc.

Momme a Japanese weight used in the marketing of cultured pearls. 1 momme=3.75 grams= 18.75 carats=75 pearl grains.

Monobromonaphthalene (x type) a liquid used as an immersion fluid in the microscopical examination of gemstones, and in certain refractive index tests. R.I. 1-66; S.G. 1-49; B.P. 279 C.

Monochlorbenzene a liquid having a value in certain refractive index tests. R.I.= 1 53.

Monochlornaphthalene a liquid having a value in certain refractive index tests. R.I.= 1 63.

Monochromatic light light of one wave-length only. In practice it is usual to employ the yellow glow of sodium vapour.

Monoclinic one of the crystal systems. See Chart 1 13.

"Montana ruby" a MISNOMER ( COMMERCIAL LABEL ) for red garnet.

"Mont Blanc ruby" a MISNOMER ( COMMERCIAL LABEL ) for rose quartz.

Moonstone see Feldspar.

Morallons a name locally used in the Colombian emerald mines for inferior stones. See Canutillos.

Morganite rose-coloured beryl, see Beryl.

Morion a black variety of smoky quartz.

Moroxite see Apatite.

Mortar a bowl-shaped vessel of agate or earthenware in which minerals are powdered by the aid of a club-shaped instrument known as a pestle. A variation consisting of a hollow steel body into which loosely fits a steel pestle, is used for reducing diamond to powder.

Moss agate chalcedony with dendritic inclusions.

Mossy stones stone rendered cloudy by fissures. Mostly referring to emeralds.

"Mother of emerald" a MISNOMER ( COMMERCIAL LABEL ) for prase.

Mother-of-pearl the iridescent nacreous material from the shells of molluscs.

Mother rock see Matrix.

"Mountain jet" a MISNOMER ( COMMERCIAL LABEL ) for black obsidian.

"Mountain ruby" a MISNOMER ( COMMERCIAL LABEL ) for red garnet.

Multi-facet diamond a term used to describe a diamond with the girdle polished or faceted. The term is American, and application has been made to use it as a trade mark.

Mussel pearls pearls obtained from the freshwater mussel. The most important species in Europe is Unio margaritifera. In the U.S.A. the genus Quadrula supplies such pearls.

Nacre the secretion from the mantle of certain molluscs consisting of crystalline carbonate of lime (CaC03) and an organic material called conchiolin. It is the iridescent material of pearl and mother-of-pearl.

Narwhal ivory an ivory obtained from the "horn" or tusk of the narwhal (Monodon monoceros).

Nassau pearls conch pearls from West Indies waters.

Natrolite R.I. 1-48 --- 1-49; S.G. 2-2; H. 5.5; White (fibrous) but other colours are known; Orthorhombic; Norway and Scotland.

Natural original surface of rough diamond sometimes left in a fashioned stone, often on the girdle.

Natural glass see Moldavite, Tektite and Obsidian.

Navette-cut a style of cutting, generally step (trap) cut with a boat-shaped outline, or rectangular with pointed ends. The style is similar to Marquise. See Cuts of stones, Chart 182.

Needle stone quartz with needle-like inclusions of other minerals, such as rutile, actinolite, etc. Also known as Sagenitic quartz or Rutilated quartz, and Venus or Thetis hair stone.

Negative crystal a cavity in a crystal having the crystal form of the host mineral. It is one form of inclusion.

Negative reading the effect shown on a refractometer when the specimen under test has a higher refractive index than the dense glass of the instrument, or higher than the R.I. of the liquid used to make optical contact.

Neolith an imitation turquoise of German origin.

Nephrite R.I. 1-60 --- 1-63 to 1-62 --- 1-65; S.G. 2-90 to 3-02; H. 6.5; Monoclinic; Colours, white, leaf-green, emerald-green, dark green; Localities, Canada, China, Siberia, New Zealand, Rhodesia, Turkestan, U.S.A.; Alternative names, Maori stone, New Zealand Greenstone, Axe-stone, and Kidney stone. It is one of the 6`Jades'9.

Neutron-treated diamonds diamonds which have had their colour altered by neutron bombardment in an atomic pile. Same as "Pile-treated diamonds".

"Nevada black diamond" a MISNOMER ( COMMERCIAL LABEL ) for obsidian.

"Nevada diamond" a MISNOMER ( COMMERCIAL LABEL ) for decoloured obsidian.

"Nevada topaz" a MISNOMER ( COMMERCIAL LABEL ) for smoky obsidian.

Nevada wonderstone a volcanic rock in stripes of red and buff. Density is about 2 53; Nevada, U.S.A.

"New jade" a MISNOMER ( COMMERCIAL LABEL ) for bowenite serpentine.

New Zealand greenstone an alternative name for the nephrite found in New Zealand.

Niccolite a pale red coloured mineral with a metallic lustre which has a H. of 5 to 51 and a density varying between 7 3 and 7 6. The crystallization is hexagonal but the mineral is always found massive. U.S.A., Canada, and Europe. Also known as kupfernikel.

Nicol prism a device for the production of polarised light, consisting of a rhomb of Iceland-spar (calcite) cut and cemented diagonally so that the ordinary ray is reflected out of the side of the rhomb and the extraordinary allowed to pass through. Two Nicols are used in a petrological microscope, the first, the polariser, below the stage, and the second, the analyser, in the tube above the objective.

Nitric acid a powerful mineral acid. One of the constituents of aqua regia.

Nitrobenzene a liquid, having a value in certain refractive index tests. R.I.= 1 55.

Non-nucleated cultured pearls Cultured pearls grown by the insertion of a piece of mantle into the mantle of the Japanese freshwater mussel (Hyriopsis schlegeli). These pearls are usually white, small and baroque or bun-shaped. Their nature may be revealed by their strong fluorescence under X-rays and by the peculiar cavities shown by an X-ray picture.

"Norbide" trade name for an abrasive made of boron carbide.

Nucleus the central mass around which matter accretes or grows. Generally used with reference to the centre of the pearl.

Nyf the name given to the skin found on some diamonds from South Africa.

Obsidian a volcanic glass; R.I. 1-50; S.G. 2-3 to 2-5; H. 5.5; Amorphous; Colour, black, red and brown. See also Moldavite.

Obus the name applied to a trap-cut stone oblong in outline but with one end brought to a point. See Cuts of stone, Chart 182.

"Occidental topaz" a MISNOMER ( COMMERCIAL LABEL ) for yellow quartz.

Octahedrite alternative name for anatase.

Octahedron a crystal form having eight faces. It may be described as being two pyramids, each formed of four equilateral triangles, placed base to base; a crystal form of the cubic system.

Odontolite or "Bone turquoise" fossil bone or ivory naturally coloured blue by phosphate of iron. S.G. 3 0 to 3 25; H. 5; Organic structure shown under lens and effervesces with hydrochloric acid. Colour, blue.

Off-colour the term applied to diamonds having a yellowish or brownish tinge.

Oil pearls "pearls" cut from the sea snail shell, also called Antilles pearls.

Oligoclase see Feldspar.

Olive oil an oil having a value in certain refractive index tests. R.I. = 1-47.

Olivine the mineralogical name for the iron magnesium silicate, the gem variety of which is Peridot. Olivine, often misspelt olivene, is a MISNOMER ( COMMERCIAL LABEL ) for demantoid garnet.

Once the weight the term used in the "base" system of pearl price calculation for 1/- base.

Ontario moonstone peristerite variety of feldspar from Ontario.

Onyx see Chalcedony.

Onyx Marble name for banded calcite.

Opal a silica gel; R.I.1-44 to 1-47; S.G. 1-95 to 2-20; H. 5 to 6.5; Varieties:

Precious opal showing good play of colour White on white ground ---- Black on black ground.

(1) Harlequin opal has patches of colour of a regular size.

(2) Fire-opal; semi-transparent of orange to red colour.

(3) Opal matrix is opal cut showing some of the ironstone matrix.

(4) Girasol transparent blue-white with a red play of colour.

(5) Lechosos opal; a variety showing a deep green play of colour. Prase opal is coloured green.

(6) Memlite or Liver opal is grey or brown.

(7) Milk opal is yellowish, bluish-white, or white in colour.

(8) Mexican Water opal; a clear colourless or yellowish opal showing a play of colour.

(9) Moss opal; opal with dendritic inclusions.

(10) Resin opal; yellow in colour with a resinous lustre.

(11) Wood opal; an opal pseudomorph after wood.

(12) Hyalite is a colourless, glass-like opal.

(13) Hydrophane is a dehydrated opal which becomes opalescent when placed in water.

(14) Cacholong is a white porcelain-looking type.

Localities, (Precious Opal) Hungary, Australia; (Fire Opal) Mexico.

Opal doublet a composite stone consisting of a thin slice of opal backed with a piece of "potch opal", or with a piece of black onyx or special black glass called "opalite".

Opal, treated opal which has been stained with a black carbonaceous compound to give a black colour to an otherwise white opal.

Opalescence a reflection of a milky or pearly light from the interior of a mineral. Also used by some as an alternative to Iridnescence.

Opalite a name applied to some types of common opal.

Operculum the name applied to the calcareous disc found at the head of certain univalve molluscs and used by the animal as a door to close the aperture of the shell. The opercula from the shellfishes Turbo petholatus, found in the South Seas north of Australia, have a limited application in jewelry.

Ophicalcite the mineral name for serpentinous marble, such as Connemara marble.

Optical sign in addition to the classification of minerals into optically "uniaxial" and "biaxial" (q.v.), a further subdivision is made into those which are optically "positive" and those which are optically "negative". The conventions are as follows: (a) with uniaxial minerals, those in which the extraordinary refractive index is greater than the ordinary are said to be positive, while those in which the extraordinary index is the lower are termed negative. (b) in biaxial stones, those in which the intermediate index, 6, is nearer to the lowest index, a, than to the greatest index, y, are termed positive, while those in which ,B is nearer y are termed negative. Examples: tourmaline is uniaxial negative; topaz is biaxial positive.

Optic axes directions of single refraction in doubly refracting stones. In the tetragonal and hexagonal systems there is one such direction and such crystals are termed uniaxial; rhombic, monoclinic and triclinic crystals have two directions of single refraction and are termed biaxial.

Optic axial angle the acute angle subtended by the optic axes in biaxial crystals, usually denoted as 2V, or 2E as seen in air.

Orbicular the term used for a mineral or rock containing numerous spherules solidly encased, e.g., orbicular jasper.

Ordinary ray a ray in a doubly refracting stone which behaves similarly to a ray passing through isotropic material, in that it travels with the same velocity whatever its direction in the stone. This ray is only possible in crystals of the tetragonal, hexagonal and trigonal systems (uniaxial crystals).

Organic produced by vital processes. In chemistry the compounds of carbon are termed "organic" compounds.

"Oriental" (Emerald, Amethyst, Topaz, etc.) a prefix sometimes used to describe corundum, having similar colour to the stone described in the second part of the name viz.: "Oriental Amethyst" is violet sapphire. A most undesirable MISNOMER ( COMMERCIAL LABEL ).

"Oriental Alabaster" see Egyptian alabaster.

"Oriental emerald" this term, besides being used as an incorrect name for the green corundum, has also been applied to the green, chlorospinel type of natural spinel. Whether used for corundum or spinel the term is definitely a MISNOMER ( COMMERCIAL LABEL ).

Orient of pearl the iridescent surface sheen of gem pearl. It is due to the combined effect of a play of colour due to interference of light at thin films, the thin plates of the nacreous layer; and to diffraction from the fine edges of the plates.

Ornamental stones a term used for those minerals, which, through lack of transparency, own their beauty solely to their body colour and are used mainly for small carvings and objets d'art. Such stones are Malachite, Lapis lazuli, marble, etc.

Orthoclase monoclinic feldspar, which see.

Orthorhombic a crystal system for which the term Rhombic is an alternative.

Orthotoluidine a liquid having a value in certain refractive index tests. R.I.= 1 57.

Osmium S.G. 22 5; a metal of the platinum group having the distinction of being the heaviest known metal and a very high melting point (2,700 C. about). Osmium has no annlication in jewelry.

Oxidation the chemical change by which oxygen is added to an element or compound.

Oxides compounds of oxygen with another element. Examples are Coruridum and Quartz.

Pacific Cat's-eye alternative name for operculum. See Operculum and Chinese cat's eye.

Padparadschah a name applied to a peculiar orange-pink variety of natural gem corundum, and also used, with variations in spelling, for synthetic corundum of similar colour.

Pagoda stone a type of translucent agate which when cut shows markings resembling a Burmese pagoda, in opaque white bands which appear to float in the colourless translucent body material. Also used for agalmatolite.

Painite Calcium boro-silico aluminate. R.I. 1-79 --- 1-82; S.G. 4-01; H. 7.5; Hexagonal; Burma.

Painted stones a fraudulent method used to improve the colour appearance of yellowish or brownish diamonds by applying a film of blue colour to the base facets. The best method of detecting such a fraud is to soak the stone in spirits or to boil the stone in sulphuric acid. Pale colour emeralds and rubies have had their colour enhanced by coating the back facets with coloured varnish or dye. The detection method suggested for diamond will also apply to these stones.

Palladium S.G. 114; a metal of the Platinum group, similar in colour to platinum and used, owing to lower cost, for jewelry and the arts instead of platinum.

Parian marble a pure white marble.

Parti-coloration the effect seen in crystals which are coloured dierently in different parts. Stones cut from such crystals will show two or three colours, such as red one end with green the other, and, maybe, colourless in centre. Such stones are termed Parti-coloured. Parti-coloration is often shown in corundum and tourmaline.

Parting (or False cleavage) a direction of weakness in certain crystals (e.g., corundum) due to lamellar twinning.

Paste name given to the glass used for imitation gemstones. See also Strass.

Pavilion facets the long five-sided facets on the pavilion, or base, of a brilliant-cut stone.

Pearl a concretion consisting of concentric layers of an organic material (conchiolin) and crystalline calcium carbonate in the form of aragonite arranged radially around a small body as nucleus. S.G. 2 65 to 2 78; Localities, Persian Gulf, Ceylon (Gulf of Manar), north-west coast of Australia, Philippine Islands, coast of Venezuela, Gulf of Mexico, Pacific Ocean Islands, rivers of U.S.A., Europe and China.

1 Baroque; a pearl of irregular shape.

2 Black; a pearl of bronze or gunmetal tint from the Gulf of Mexico and certain other localities.

3 Blister; pearl formed attached to the shell of the mollusc.

4 Blue; natural pearl in which the nucleus is mud, clay or organic material.

5 Cultured; a pearl in which the nucleus (of mother-ofpearl) had been artificially inserted in the oyster.

6 Freshwater or Mussel; a pearl found in the Pearl Mussel (Unio margaritifera) in rivers of Scotland, Europe and U.S.A.

7 Pink; obtained from a univalve shellfish, the Indian conch or "Abalone". A non-nacreous pearl.

Pearl sac the term applied to the bag of epithelial cells (part of the mantle) which surrounds the pearl in the flesh of the oyster. The pearl sac is an essential condition for the formation of cyst pearls, either natural or cultured.

Pearls, Calculation of prices; see Chart.

Pectolite R.I. 1-59 --- 1-63; S.G. 2-74 to 2-88; H. 5; Monoclinic (massive); Greyish white; U.S.A., Italy and Scotland.

Pegmatite a coarse-grained quartz-feldspar igneous rock.

Pendeloque a style of cutting. A drop-shaped stone. See Styles of cutting, Chart 18 l.

Pennyweight, Pwt or Dwt 1 penny wt.= 5% of an ounce "Avd" 20 to the ounce. 1/20thof ounce, 24 grains, 0.05 ounces, 0.0648 grams.

Periclase a magnesium oxide mineral now produced synthetically under the name "lavernite". Cubic. H. - 5; S.G.=3 55 to 3 60 and R.I. = 1 73.

Peridot R.I. 1 654-1 689; S.G. 3 34; H. 61 to 7; Rhombic; Colours, oil-green, brown; Localities, Red Sea, Burma, Ceylon, U.S.A. and Norway. (Mineralogical name of the species is Olivine).

Periostracum the name given to the outer dark horny layer of conchiolin forming the outside of the shell of the oyster.

Peristerite a variety of albite feldspar showing a bluish opalescence somewhat like labradorite.

Petalite R.I. 1-50 --- 1-516; S.G. 2-39 to 2-46; H. 6; Monoclinic (crystals rare, material usually massive); Colourless or white; Sweden and Maine, U.S.A.

Petroleum a hydrocarbon liquid having a value as an immersion liquid in certain refractive index tests. R.I.= 1-45.

Petrology the study of the mineral constitution of rocks and their mode of formation.

Petrological microscope see Microscope.

Phantom Crystals see Ghost crystals.

Phenakite R.I. 1 651-1 666 to 1 653-1 668; S.G. 295 to 297; H. 71 to 8; Trigonal; Colours, colourless, yellowish and pale pink; Localities, Ural Mountains (Russia) and North and South America.

Phenyldi-iodearsine a highly refractive liquid (R.I. 185) used for making optical contact between stone and dense glass of refractometer, and as an immersion fluid. It is poisonous and has a blistering action on the skin.

Phosphorescence is the continuance of fluorescent light after the exciting radiations causing the luminescence have ceased. See also Fluorescence.

Phosphoroscope an apparatus for measuring the duration of phosphorescence.

Photomicrograph a photograph of the view of a subject as seen through a microscope. A special camera is used which fits on the tube of the microscope in place of the eyepiece, or over the eyepiece.

Piezo-electricity the property certain crystals (notably quartz and tourmaline) possess of inducing a charge of electricity in themselves when pressure is applied along certain directions in the crystal.

Pil - Treated diamonds Diamonds coloured by bombardment with neutrons from an atomic pile (atomic reactor). As in cyclotroned diamonds the colours are green and after heat treatment brown. These stones are not radio-active and are coloured throughout.

Pinacoidal face a crystal face that is parallel to two of the crystal axes.

"Pink moonstone" a name erroneously applied to the opalescent pink variety of scapolite.

Pique the term applied to diamonds which have inclusions obvious to the naked eye.

Pistacite see Epidote.

Planes of symmetry those planes in an ideally developed crystal which divide it in such a way that one side of each plane is the mirror image of the other.

Plasma see Chalcedony.

Plastics an omnibus term used for a number of synthetic substances, generally resin-like, which can be moulded by heat and pressure. See Chart 176.

Platinum S.G. 21-5; a hard and ductile grayish-white metal used for mounting gemstones, particularly diamonds. The metal has a high melting point (1755 C.) hence is employed where heat resistance is required.

Play of colour iridescence due to interference of light impinging on thin films having a different refractivity to the surrounding mass. Seen in precious opal, labradorite, etc.

Pleochroism the optical effect of gem stones of having different colors depending on which optic axis that the viewer is looking at.

Pleonaste see Spinel.

Point the term used in describing the weight of diamonds or other gems under one carat. The "point" is 0.01 carat so that a fifty point diamond weighs 0.50 carat, or half a carat. A seventy point stone weighs 0.70 carat.

Polariscope an instrument consisting of two units for the production of plane polarised light arranged with a rotating stage between them. The lower unit (the unit may be a Nicol prism, an Ahrens prism, a "Polaroid" disc or may be a mirror arranged at the requisite angle) is termed the "polariser" and the similar unit above the stage the "analyser". The polariser, and sometimes the analyser, are capable of being rotated at right angles to the optical axis of the insku. meet. The polariscope is used to examine substances in parallel and convergent light. All petrological microscopes are fitted with the device.

Polarised light light which is vibrating in one plane only, whereas with ordinary light the plane in which the vibrations may take place are infinite in number. In all rays of light, polarised or unpolarised, the vibrations are at right angles to the directions the light is travelling. Doubly refracting crystals not only resolve the light that passes through them into two rays but completely polarise them in planes at right angles to one another.

Polaroid sheets of plastic material containing ultra-microscopic crystals of quinine iodosulphate which have the property of transmitting only one polarised ray, the other being almost entirely absorbed by the crystals. May be used in place of the more expensive Nicol prisms.

Pollucite R.I. 1-51; S.G. 2-86; H. 6.5; Cubic; Colourless; Localities U.S.A. and Isle of Elba.

Polymorphism the term used where minerals which have the same chemical composition crystallise in different forms, e.g., Rutile; Anatase and Brookite all correspond to the formula TiO2 but each appears in different crystal form. See Dimorphism.

Polystyrene a plastic. A polymerisation product of vinyl benzene. R.I. 1-59 to 1-67; S.G. 1-05; H. 2.5. Dissolves in hydrocarbon liquids, e.g., benzene, bromoform, etc.

Polysynthetic twins contact twin crystals producing a number of very thin plates (laminate or lamellae). Each adjacent plate is in reverse order in alternate plates they are in the same order. Sometimes called repeated or lamellar twinning. Weakness along the cGntact planes gives rise to "Parting" or "False cleavage".

Porcelain a type of fine earthenware (china) made from china-clay (kaolin) sometimes used for making opaque imitation stones, such as turquoise. S.G. 2 1 to 2 5.

Porphyries types of igneous rocks which show comparatively large and well-formed crystals embedded in a fine "rained ground mass. Red and green coloured porphyries were much used in Greco-Roman times.

Potch opal Miners term for opal which does not show the play of colour seen in precious opal.

Powder, Diamond see Diamond Powder.

Prase green massive quartz.

Prasiolite the name given to the green quartz due to the heat treatment of amethyst.

Precious stones A term usually confined to such stones as the diamond, ruby, sapphire, emerald, and perhaps black opal. It is an older term with little effective meaning because many other stones are more expensive than these, especially if these are of low quality and the other stone if of fine quality.

Prehnite R.I. 1-62 --- 1-65; S.G. 2-80 to 2-95; H. 6 to 7; Rhombic; Colour, green (may resemble jade) and brown; Localities, France, U.S.A. and Australia.

Pressed amber amber which has been made up of small pieces welded together by hydraulic pressure under heat; a true reconstruction. Alternatively called Ambroid.

Princess Cut a square cut of diamond that uses back facets that are more like a brillant cut diamond; an old and discontinued name for the profile cut which is used for diamonds and other stones.

Profile cut a style of cutting for diamond (and has been used for other stones) in which flat slices of diamond are cut with grooves on the reverse side. Said to give economy of material and facilitates setting by the fact that all stones are cut to definite sizes. Previously known as the Princess cut.

Proustite R.I. 2-7 --- 3-0; S.G. 5-57 to 5-64; H. 2.5; Trigonal; Red (darkens on exposure to light); Central Europe, Chile, France, Mexico and U.S.A.

Pseudomorph the term used to describe a mineral which has been altered from another, but in which the original form has been retained.

Pseudophite a hydrated silicate of aluminium and magnesium; R.I. 1-576 - 1-579; S.G. 2-6 to 2-85; H. 2.5; Colour, green; Localities, Switzerland, Italy, Styria (Austria) and Scandinavia. Known also as "Styrian Jade".

Psilomelane a manganese mineral which when cut and polished resembles hematite and has been used similarly.

Pumice powder a highly cellular, glassy volcanic rock or lava used as a polishing agent.

Pumicite or Volcanic dust a natural glass or silicate atomised by volcanic explosions. The material is similar in composition to pumice powder and like this latter material is used for polishing purposes.

Punamu the Maori name for the jade from New Zealand. See Nephrite.

Purple gold may be obtained by the use of aluminium as an alloy. This alloy, however, is too brittle for use in jewelry.

Putty powder a polishing medium consisting of tin oxide (about 85 to 90%) with the remainder lead oxides.

Pyknometer see Specific Gravity Bottle.

Pyralspite series the name applied to the isomorphous series of garnets which contain the pyrope/almandine/spessartite group.

Pyrite (pyrites) S.G. 4-84 to 5-10; H. 6..5; Cubic; Colour brass-yellow. Pyrite is the "Marcasite" of jewelry.

Pyro-electricity the property of certain crystals (particularly tourmaline) of acquiring an electric charge when they are heated.

Pyrope see Garnet.

Pyrophyllite R.I. mean 1-58; S.G. 2-8; H. 1.5; White, grey or greenish and resembles soapstone. The crystallization is orthorhombic but the mineral is always massive. Russia, Brazil, U.S.A. and South Africa where the grey material has been marketed under the name "wonderstone".

Pyroxene The name applied to a group of minerals whose physical and optical characters serve to link them together. They are closely analogous chemically to the amphibole group (which see). The gem members of the pyroxene minerals are; enstatite, diopside, jadeite and spodumene.

Quartz R.I. 1-544 --- 1-553; S.G. 2-65 to 2-66; H. 7; Trigonal; Colours, colourless (Rock Crystal), violet (Amethyst), yellow (Citrine), brown (Cairngorm), pink (Rose Quartz), green (Prase), green chatoyant (Cat's-eye), yellow chatoyant (Tiger's-eye), blue (Siderite), brown, yellow, red or green with scales of mica (Aventurine), colourless with cracks showing prismatic colours (Iris), colourless with included acicular crystals of rutile, etc. (Sagenitic quartz, Rutilated quartz, Venus' or Thetis hair stone). Quartz, owing to its piezo-electric effect, is used in radio frequency control, etc. See also chalcedony.

Quartz glass fused quartz which thereby loses its crystalline character and becomes a glass. S.G. 2 21; R.I. 1 46.

Quartzite a metamorphic rock made up of irregular interlocking grains of quartz. Often contains flakes of green mica or a reddish iron mineral. Such types with coloured or spangled inclusions are known as aventurine quartz.

Quoin facets the name applied to the other four facets adjacent to the bezel facets, on the crown in a brilliant-cut stone. These eight facets, the quoins and bezels are often called collectively the bezels or kites.

"Radiant" suggested, but undesirable name for the synthetic colourless spinel. Another of these fancy pseudonyms for this synthetic spinel is Corundolite.

Radiant cut a cut of diamond that has a rectangular outline like an emerald cut but the facet placement more approaching the arrangement found on a brillant cut.

Radium diamond,71 a MISNOMER ( COMMERCIAL LABEL ) for smoky quartz.

Radium-treated diamonds Diamonds when irradiated, that is kept adjacent to radium bromide for some time, assume a Green colour which is fairly permanent. This is due to bombardment by the alpha particles of the radium. These treated stones are radio-active and can be distinguished from natural stones of a green colour by their ability to fog a photographic film.

Reconstructed stones stones made by fusing together small pieces of real crystal to make a larger stone, care being taken to ensure crystallisation and better colour obtained by the addition of colouring oxide. Was usually confined to the making of ruby. Now superseded by the synthetic gem.

Red gold an alloyed gold showing a redder colour than the normal yellow gold. It is often used for contrast and is made by alloying with copper alone, or copper and silver in the proportion of three to two.

Reduction the term applied to the chemical change in which the oxygen or other non-metal is taken away from a compound.

Reflection of light a ray of light striking a polished surface is reflected from it in accordance with the following laws: (a) the angle of reflection is equal to the angle of incidence. (b) the incident ray, the reflected ray and the normal at the point of incidence are in the same plane.

Reflectometer alternative, and incidentally more correct name for refractometer.

Refraction of light a ray of light passing from a medium to one optically denser is bent towards the normal, and similarly in passing into a medium less dense is bent away from the normal. This bending is in accordance with definite laws. See Snell's Law, Chart 128.

Refractive index the speed of light in a gem stone as a relation to the speed of light in a vacuum. Used as a non-distructive test to identify gems.

Refractometer an optical instrument arranged to show the critical angle of total reflection as a shadow edge, on a scale calibrated in refractive indices. In the case of the better class instruments used in laboratories the position of the shadow edge is read oR on a graduated arc. There are different types which may use highly refractive glass, synthetic spinel or diamond for the prism or hemisphere.

Relative density alternative term for specific gravity. More usually used for liquids. See Density and Specifie gravity, Chart 117.

Resin Opal see Opal.

Rhinestonea name applied to the transparent paste which, colourless in the main, shows patches of various colours.

Rhodium S.G. 12-44; a white metal, nearly the same colour as platinum, used for plating. This rhodium plating process is used to improve the colour and general appearance of gem-set jewelry.

Rhodizite a borate of aluminium and caesium; R.I. 169; S.G. 3 40; H. 8; Cubic; Colours, pale green and pale yellow; Locality, Madagascar. Very rare.

Rhodochrosite R.I. 1-597 --- 1-817 to 1-605 --- 1-826; H. 4; S.G. 3-45 to 3-70 for the rose red material, rising with increase of iron; Trigonal; Colour, rose-red; presence, by replacement of iron, calcium and magnesium, introduces shades of yellow and brown; Localities, Hungary, Saxony, Colorado U.S.A. and Argentina. Inca rose and Rosinca are fancy trade names for this material.

Rhodolite see Garnet.

Rhodonite R.1. 1-71--- 1-73 to 1-72 --- 1-75; S.G. 3-5 to 3-7; H. 5 to 6; Triclinic; Colours rose-red and pink (opaqued; Localities, Russia and U.S.A. small clear crystals found at Wermland in Sweden are occasionally cut.

Rhombic (Orthorhombic, Trimetric) one of the crystal systems. See Chart 1 13.

Rhombic dodecahedron the geometric solid with twelve rhomb-shaped faces. A crystal form of garnets and some diamonds.

Rhombohedral see Trigonal.

Rhombohedron a solid bounded by six rhombohedral faces; a common form in trigonal crystals.

Ribbon jasper jasper in which the colours run in stripes.

River pearl natural pearl from freshwater mussels.

Rock-crystal see Quartz.

Rose quartz see Quartz.

Roses the name applied to small diamonds cut in the rose-cut style.

Rosinca a fancy name for Rhodochrosite.

Rosolite see Xalostocite.

Rottenstone a soft, friable, very finely textured earthy mass used as a polishing compound. The material is a residual product from the weathering and decaying of a siliceous, argillaceous limestone.

Rouge the red oxide of iron used as a polishing medium. Green rouge is chromium oxide.

Roumanite Roumanian amber. See Amber.

Royal-cut A modern cut for diamond using a greater number of facets.

"Rozircon" a MISNOMER ( COMMERCIAL LABEL ) for the rose coloured synthetic spinel.

Rubasse a spangled red variety of quartz or rock crystal coloured by the inclusion of minute blood-coloured scales of oxide or iron. The natural stone is rare and is found in Brazil. This stone may be imitated by cracked quartz which has had red dye introduced along the cracks.

Rubbish a mining classification for diamonds which can only be used for making powder.

Rubellite see Tourmaline.

Rubicelle see Spinel.

Ruby see Corundum.

"Ruby, Adelaide" a MISNOMER ( COMMERCIAL LABEL ) for red garnet.

"Ruby, Arizona" a MISNOMER ( COMMERCIAL LABEL ) for the red garnet.

"Ruby, galas" a MISNOMER ( COMMERCIAL LABEL ) for the pale red spinel.

"Ruby, Bohemian" a MISNOMER ( COMMERCIAL LABEL ) for red garnet.

"Ruby, Brazilian" a MISNOMER ( COMMERCIAL LABEL ) for rose-red (fired) topaz.

"Ruby, Cape" a MISNOMER ( COMMERCIAL LABEL ) for the red garnet from South Africa.

"Ruby, Colorado" a MISNOMER ( COMMERCIAL LABEL ) for red garnet.

"Ruby, Montana" a MISNOMER ( COMMERCIAL LABEL ) for red garnet.

"Ruby, Siberian" a MISNOMER ( COMMERCIAL LABEL ) for rubellite. See Tourmaline.

"Ruby Spinel" a MISNOMER ( COMMERCIAL LABEL ) for red spinel.

Ruin marble a yellow marble with sections of a brown colour, due to the infiltration of iron.

Ruthenium S.G. 12-3; a metal which, through belonging to the platinum group, has so far had no application in jewelry.

Rutile R.I. 2-62 --- 2-90; S.G. 4-2 to 4-3; H. 6 to 6.5; Tetragonal; Colours, blood red, reddish-brown and black; Localities, Russia, Scandinavia, Italy, France, U.S.A., Switzerland, Madagascar and Brazil. Rutile is made synthetically by a modified flame fusion process. The colours produced are yellowish-white, yellow, orange, red and blue, the latter are very rare. They have similar constants to the natural rutile.

Rutilated quartz clear rock-crystal with included crystals of rutile (see also Sagenitic Quartz Venus' hair stone).

Sagenitic quartz rock crystal with included crystals of rutile. See Quartz.

Sapphire see Corundum.

"Sapphire, Brazilian" a MISNOMER ( COMMERCIAL LABEL ) for blue tourmaline, or similar coloured topaz.

"Sapphire, Lux" a MISNOMER ( COMMERCIAL LABEL ) for iolite.

"Sapphire, Lynx" a MISNOMER ( COMMERCIAL LABEL ) for the pale blue shade of iolite.

"Sapphire, Spinel" a MISNOMER ( COMMERCIAL LABEL ) for blue spinel. See Spinel and Ruty spinel.

"Sapphire, Water" a MISNOMER ( COMMERCIAL LABEL ) for the dark blue shade of iolite.

Sapphirine a term variously applied to the blue spinel or to the blue quartz. It is a name better dropped.

Sard and Sardonyx see Chalcedony.

Sark Stones Amethyst found in the island of Sark in the Channel Islands. Very little found to-day.

Satin-spar fibrous white or pink calcite, or similar gypsum.

Saussurite a decomposed feldspar; S.G. 3 2 (about); H. 61 to 7; Colour, greenish-gray to white; Locality, Switzerland. Sometimes simulates jade.

Sawing a method whereby a diamond may be cut into two or more parts in directions other than the cleavage directions. It is carried out by the use of a diamond saw, a phosphorbronze disc fed with diamond powder and olive oil, and running at about 5,000 r.p.m. The same process is used also for the cutting of the less hard gemstones.

"Saxony diamond" a MISNOMER ( COMMERCIAL LABEL ) for colourless topaz.

Scapolite an isomorphous mixture of marialite and meionite R.I. (blue) 1-544 --- 1-560 (pink and yellow), 1-548 --- 1-570; S.G. 2-61 to 2-70; H. 6.5; Tetragonal; Colours yellow, pink (chatoyant), blue (chatoyant); Localities, Burma Madagascar and Brazil.

Scaife a name applied to the cast-iron lap, which is 10" to 12" in diameter and used in diamond polishing. It must have an open porous structure and is provided with a paste of olive oil and diamond dust upon which diamonds are polished.

Scheelite S.G. 5-9 to 6-1; H. 4 to 5; Tetragonal; White yellow, brownish to orange. Most of the gem material comes from the U.S.A. and Sonora, Mexico. Scheelite does not fluoresce under the long-wave ultra-violet lamp but glows with a blue colour under the short-wave lamp. Also made synthetically.

Schiller the silver shimmer of light seen just below the surface of a stone and due to lamellar twinning, best seen in bronzite and moonstone.

Schist a foliated and fissile metamorphic rock of layers of different minerals.

Schorl black tourmaline.

Scisscor-cut a variation of the trap cut in which the side facets surrounding the table are cut as elongated triangular facets. Also called Cross-cut. See Cuts of stones, Chart 181.

"Scotch Topez" a MISNOMER ( COMMERCIAL LABEL ) for citrine, smoky quartz or cairngorm.

Seed pearls very small pearls.

Selenite the crystallised form of gypsum, generally found in well-formed monoclinic crystals. See Gypsum.

Semi-precious stones An unsatisfactory term for all gemstones other than those mentioned as being "Precious stones". It is a term now discontinued by general consent.

Senaille the name applied to small chips of diamond with irregularly polished facets set into jewelry. Often erroneously called `'roses".

Sepiolite see Meerschaum.

Serpentine R.I. 1-50 to 1-57 (mean); S.G. 2-50 to 2-65; H. 2.5 to 4; Monoclinic; Colours, greens, etc.; varieties, Williamsite, Bowenite, and Verd Antique may resemble jade. Bowenite is unusual in having hardness 51 to 6.

Setting edge an alternative name for the girdle.

Shagreen a material sometimes used as a covering to cigarette cases, lighters, mirrors, brushes and small boxes, etc. Originally made from the skin of the wild ass, which even now, with that of the horse and camel, provides much of the material. Shagreen is now generally derived from the skins of certain sharks and ray fishes. The material is dyed to suitable colour, generally green, blue, red and black.

Shatter marks see Fire marks.

Shattuckite R.I. about 1-80; S.G. about 3-8; H. 3: to 4; Monoclinic (massive); Blue; U.S.A. Its status as a mineral species has been disputed.

Sheen a shimmering effect, due to reflection of light from a position inside the stone, in contradistinction to "lustre" which is a surface reflection.

Shell-marble a marble consisting mainly of fossil shells.

Shipley colorimeter an instrument for the colour grading of diamonds.

"Siam aquamarine" a MISNOMER ( COMMERCIAL LABEL ) for the heat-treated bluish green zircon.

"Siberian ruby" a MISNOMER ( COMMERCIAL LABEL ) for rubellite. See Tourmaline.

Siberite a name used for violet tourmaline.

Siderite blue quartz.

Silica-glass R.I. 146; S.G. 221; H. 6; Amorphous; Colour, pale greenish yellow; Locality, Libyan Desert. A celestial origin is suggested by some authors, others regard it as sandstone fused by intense heat of basaltic intrusions. See also Quartz glass.

Silicates formerly thought of as salts of various silicic acids. Now considered as compounds in which the structure is largely determined by the silicon-oxygen linkages which form the framework of the crystals. Examples are; Beryl, Spodumene, Zircon.

Silk the whitish sheen seen in some corundums and due to the presence of vast numbers of microscopically small canals reflecting the light. "Pseudo-silk" is understood to be due to microscopic acicular crystals of titanic iron and is often observed in Siam stones.

Sillimanite see Fibrolite.

Simetite Sicilian amber, see Amber.

Silver S.G. pure 10-5, standard 10-31; a lustrous white metal, malleable and ductile used in jewelry and for ornamental and utility articles. Standard silver has 925 parts of pure silver with 75 parts of alloy to the 1,000.

Silver Cape a classification of diamond.

Sine of angle the ratio between the side opposite that angle and the hypotenuse. The hypotenuse being the side opposite the right angle of a right angled triangle.

Sinhalite R.I. 1-67 --- 1-71; S.G. 3-47 to 3-49; H. 6.5; Orthorhombic; Yellow and brown; Ceylon, Burma. A stone known for years as brown peridot and not discovered as a new mineral until 1952.

"Sira" trade name for an abrasive, made of aluminium oxide artificially manufactured.

Skew facets see Crossfacets.

Skiagram an X-ray shadow photograph produced by radiations which have passed through the substance under examination. It depends upon the differing degrees of transparency of the various parts of the substance and is the method used in medical diagnosis. The method is used for the detection of cultural pearls.

Skill facets are the eight small triangular facets around the girdle edge of the crown in the brilliant cut. They are adjacent to the cross or skew facets. The combined 16 facets, the eight cross and the eight skill, are often collectively known as break facets, half facets or halves.

Slate; an argillaceous sedimentary rock of grey colour. It is used for the so-called "black marble", which early in this century was ated by heat.

Sparklite a name which has been applied in the U.S.A. to the colourless (fired) zircon.

Specific gravity The weight of a gem stone as a relation of the same volumn of water. Arrived at by weighing the stone out of water and weighing the stone in water then expressing the Specific Gravity as a relationship of these two weights. Used as a non-distructive way of identifing gemstones.

Specific gravity bottle or Pyknometer a small flask fitted with a ground glass stopper pierced lengthwise with a capillary opening so that the bottle can be filled to a definite mark. It is used for the determination of the density of liquids, powders and small fragments by the direct weighing method.

Spectra plural of spectrume.

Spectrograph an instrument arranged for the production of spectra as in a spectroscope, but having a camera fitted in place of the telescope of the latter instrument so that a plate exposed in it will record, after development, the spectral lines (in bright line emission spectra) or the sections and bands preferentially absorbed (absorption spectra). Quartz prisms and lenses are used in order to obtain results in the ultra-violet region.

Spectrolite labradorite feldspar found in Finland.

Spectrometer an instrument used for the measurement of the spectrum. There are various types which may either allow the wave-length to be read off directly or to be arrived at by computation. The most usual types used in gemmological work are the direct vision direct reading types (Beck), and the table instruments consisting of a fixed collimating tube, to parallelise the light, and fitted with an adjustable slit, a graduated circular table upon which the crystal or stone is mounted, and lastly a telescope which may be moved round the table and to which a vernier is attached in order accurately to measure the angles. This is the instrument generally used in determining refractive index by the method of minimum deviation.

Spectroscope an instrument which analyses a ray of light into the spectrum colours, and may be one of two types:

(A) the prism type, where dispersion is effected by the different refringence of the glass prism for rays of different wave-length (colour).

(b) diffraction grating type, where a grating of many fine parallel lines performs a similar function by a process of interference the effects being viewed by a lens system (a short telescope).

Spectrum, Absorption the field seen in a spectroscope (or photograph, if spectrograph is used) when a source of white light from an incandescent solid is viewed by the instrument after suffering absorption by transmission through or reflection from a gemstone or other medium. Owing to the absorptionoflight by the medium being examined,darkbands or lines indicating the wave-lengths absorbed, may be seen across the otherwise continuous spectrum. This often gives useful information of a diagnostic nature. See Chart 147.

Spectrum, Emission the field seen in a spectroscope (or photograph if spectrograph is used) when a source of light is examined by the instrument. It may be one of three kinds:

(a) Continuous; the familiar rainbow colours, a continuous graduation of light passing imperceptibly through red to violet. It is produced by incandescent solids.

(b) Swan; a fluted or banded formation of bright lines, each of which appears to be sharp towards the side of longer wave-length and to fade away on the opposite side (actually a group of very fine lines comparatively far apart on one side and crowded closer and closer together towards the other until they are so dense that they appear one line at the heady; produced by the incandescent vapours of compounds.

(C) Bright Line; a number of sharp and bright vertical lines, and are the spectra of glowing vapours of elements. These spectra serve to identify the elements present in a substance (Spectrum analysis). See Chart 155.

Specular iron ore a variety of haematite, which see, occurring in rhombohedral crystals, black in colour, and with a metallic splendant lustre.

Speed of light is approximately 186,000 miles per second in air (300,000 kilometres per second). To find the speed of light in any other medium it is necessary to divide 186,000 by the refractive index of the medium.

Spessartite, Spessartine the manganese aluminium garnet. See Garnet. The form spessartite has also been used by petrologists for a certain type of rock.

Sphalerite mineralogical name for zinc blende.

Sphene R.I. 1-888 --- 1-917 to 1- 914 --- 2-053 S.G. 3-45 to 3-56; H. 5 to 5.5; Monoclinic; Colours, yellow, green, brown and grey; Localities, Switzerland, Mexico and Burma.

Spherulites (Spherules) Rounded aggregates or rosettes of very fine needle-like crystals radiating from a common centre producing ball-like inclusions which are circles when cut across and polished. An example is orbicular Jasper.

Spinel R.I. 1-714 to 1-736; S.G. 3-58 to 3-75 (intermediate Mg-Zn type may reach 4-06; H. 8; Cubic; Colours, deep red, rose red and pink, orange-yellow (Rubicelle), purple, blue, grass-green (Chlorospinel), dark green to black (Ceylonite or Pleonaste); S.G. to 3 90; Localities, Ceylon, Siam, Burma, U.S.A.,etc. See Gahnospinel.

Spodumene R.I. 1-66 --- 1-675 to 1-66 ---1-679; S.G. 3-17 to 3-23; H. 6 to 7; Monoclinic; Colours, yellow, yellowish-green, ice-green, grass-green (Hiddenite) and lilacpink (Kunzite); Localities, U.S.A., Brazil and Madagascar.

Stag horn see Deer horn.

Stained chalcedony see Artificial treatment of gemstones, Chart 178.

Stained jasper see Artificial treatment of gemstones, Chart 178.

Stalactite a tapered column of mineral matter found pendant from the roof of a cavern and which has been deposited by dripping water.

Stalagmitic formation a cavern formation the opposite to a stalactite. It may cover the floor of the cavern or may rise as a column from the floor underneath a stalactite. In some cases the two may join to produce a column.

Star facets the name applied to the eight triangular facets which surround the table in the brilliant cut.

"Starilian" Trade name for strontium titanate.

"Starlite" named proposed, in U.S.A., for the blue zircon (heat treated).

Star stones see Asterias.

Staurolite R.I. 1-7 --- 1-75 to 1-75 --- 1-76; S.G. 3-4 to 3-8; H. 7 to 7.5; Rhombic; Colour, reddishbrown; Localities, Switzerland and South America. Twinned crystals often in the form of a cross (Cross-stone or Fairystone).

Steatite (Soapstone) R.I. 1-5 --- 1-59; S.G. 2-6 to 2-8; H. 1. may be higher owing to impurities; Monoclinic; Colours, yellow, brown and reddish. Used for carvings.

Step cut a gem cut in which the facets are parallel with the sides of the table.

Stibiotantalite R.I. 2 39-2 46; S.G. 7 4; H. 51 to 6; Orthorhombic; Brownish-yellow; U.S.A.

Stichtite R.I. mean 1-53; S.G. 2-15; H. 2.5; Trigonal (massive); Rose red to lilac; South Africa, Canada and Tasmania.

Strass a term originally used for a special type of highly dispersive flint glass used for imitation gemstones. Widely, but incorrectly used for all types of glass used as imitations. Streak; may be defined as the colour of the powder of a mineral and may be quite different from that of the mineral in mass. It is generally observed by rubbing the mineral upon a plate of unglazed porcelain termed a streak-plate. Streak marks treated with acid reagents are the standard type of test for determining the quality of precious metals, particularly gold.

Stressed figured amber cloudy amber which has during the process of clasification with rape seed oil developed stress cracks which look like nasturtium leaves. These are sometimes coloured by staining.

Striae small channels or thread-like lines, roughly parallel, seen on the surface of crystals or in the internal structure of stones, and may be described as follows:

(a) Crystal Striations; the fine lines seen on the surface of ilar and similarly-placed halves. Such a plane will divide a crystal into two parts so that one part is the mirrorimage of the other.

"Synthetic alexandrite" a MISNOMER ( COMMERCIAL LABEL ) applied to the synthetic corundum or spinel which has a greenish colour in daylight and a reddish colour in artificial light, and is made to represent the true chrysoberyl alexandrite.

"Synthetic alexandrite" a man-made true chrysoberyl alexandrite.

"Synthetic doublet" a reported composite stone consisting of a piece of synthetic red spinel or corundum and a piece of blue synthetic corundum cemented together along a vertical division making a pseudo parti-coloured stone. The method has an application for the representation of college and army colours.

"Synthetic" (garnet, tourmaline, etc.) a prefix used for synthetic corundums and spinels made in colours similar to that named, e.g., "Synthetic zircon" may be zircon colored synthetic corundum or similar coloured spinel. These terms are definitely MISNOMER ( COMMERCIAL LABEL )s. It should be remembered that there are true synthetic gemstones.

Synthetic stone a man-made creation of the same chemical and crystal structure as a natural stone. It can usually be recognized by the type and placement of inclusions which vary from the natural stone.

Syrian garnet a name for almandine garnet from Syria. See Garnet, Almandine.

Taaffeite R.I. 1-72; S.G. 3-60; H. 8; Hexagonal; Mauve; Ceylon; very rare.

Table the name applied to the large central facet on the crown in the brilliant-cut and trap-cut stones.

Talc the hydrated magnesium silicate of which soapstone is the massive variety. One of the softest minerals, it is the standard of 1 on the Mohs scale. See Steatite.

Tanzanite the name given to the transparent blue zoisite.

Tecali marble a green marble from Tecali, Pueblo, Mexico, which simulates jade.

Tektite comprehensive name for moldavite and some other natural glasses.

Television stone see Ulexite.

Templet alternative name for the bezelfacet.

Tetragonal (or Dimetric) one of the crystal systems. See Chart 112.

Tetrahedron the crystal form bounded by four equilateral triangles and belonging to the cubic system (the tetrahedron is the geometrical solid with the smallest number of faces).

Therm-luminescence a secondary light generated by certain substances when they are heated with invisible infra-red rays.

Thomsonite R.I. 1-497--- 1-625; S.G. 2-3 to 2-4; H. 5 to 5.5 Rhombic; Colours, white, red, green and yellow (mottled); Locality, Lake Superior (West), U.S.A.

Three-point see Diamond point.

Thulite a variety of zoisite.

Tiger's-eye a silica pseudomorph after crocidolite. See Quartz.

Tin-cut the term applied to moulded glass imitation stones (pastes) which have had the facets polished on a lap.

Tinstone alternative name for Cassiterite.

"Titania" trade name for synthetic rutile.

Titanite see Sphene.

Toluene a hydrocarbon liquid used for the dilution of methylene iodide and bromoform in the preparation of heavy liquids; as an immersion liquid in certain refractive index tests and, owing to its low surface tension, in place of water in direct weighing method for determining density. R.I. = 1- 49; S.G. = 0-88.

Topaz R.I. 1-607 --- 1-619 to 1-629 --- 1-637; S.G. 3-50 to 3-60; H. 8; Rhombic; Colours, yellow, blue, green and pink (generally heat treated); Localities, Brazil, Siberia, Ceylon, British Isles (rare) and U.S.A.

Topaz, Brazilian name applied to the true yellow topaz. See Topuz.

Topazolite yellow andradite. See Garnet.

"Topaz, Oriental" a MISNOMER ( COMMERCIAL LABEL ) for yellow corundum.

"Topaz, Scottish" a MISNOMER ( COMMERCIAL LABEL ) for yellow quartz.

"Topez, Synthetic" a MISNOMER ( COMMERCIAL LABEL ) for synthetic corundum or spinel of a colour resembling a topaz colour. Mostly applied to the yellow.

Tortoise-shell the horny shell of the carapace of the Hawks-bill Turtle (Chelone imbricata); R.I. 1-55 to 1-56; S.G. 1-26 to 1-35; Colours, mottled dark and light browns and yellows; Localities, Celebes, New Guinea, China, India, Africa and Australia. Tortoiseshell may be distinguished from "plastic', imitations by microscopic examination; the dark patches in the real material are seen to contain swarms of spherical reddish particles, whereas in the imitations the edges of the dark areas are more defined and lack the dot-like structure.

Total reflection the name applied to the phenomenon which occurs when a ray of light travelling through a denser medium to a rarer medium at an angle greater than the critical angle suffers complete reflection back through the denser medium. See Critical angle of total reflection.

Total reflectometers see Reflectometers.

Touchstone black and unglazed Wedgwood pottery, or basanite, a velvet-black variety of quartz, used in the testing of precious metals by applying acids to the streaks made on the touchstone.

Tourmalinated Quartz rock crystal with included crystals of, usually black, tourmaline.

Tourmaline a complex boro-silicate; R.1. 1-616-1-634 to 1-63 --- 1-652; S.G. 3-00 to 3-15; H. 7 to 7.5; Trigonal; Colours, colourless (Achroite), red and pink (Rubellite), green, blue, yellow-green, honey-yellow, violet (Siberite), dark blue (Indicolite), brown (Dravite), black (Schorl); Localities, U.S.A., Ceylon, Madagascar, Germany, Brazil and Russia.

"Tourmaline, Synthetic" a MISNOMER ( COMMERCIAL LABEL ) for synthetic spinel or corundum made in a colour to resemble tourmaline. It is generally the green colour to which the name is applied.

Transparency or Diaphaneity the term used to describe the amount of light transmitted through a substance. Degrees of transparency are classed according to the amount of light which penetrates the substance:

Transparent an object viewed through them shows outlines clear and distinct, e.g., most gemstones.

1 Semi-transparent; the outlines of an object viewed through them would be blurred but a considerable amount of light can penetrate the stone.

Translucent some light passes through but no object can be seen through the stone.

1 Semi-translucent; light is only transmitted through the edges. Opaque; allows no light to pass through.

"Transvaal jade" the light-green massive grossular garnet found in the Transvaal, South Africa, and used as a substitute for jade, has had this MISNOMER ( COMMERCIAL LABEL ) applied to it. Another MISNOMER ( COMMERCIAL LABEL ) for this material is South African jade. S.G. about 3-47 ; R.I. 1-73 ; H. 6.5.

Trap-cut name applied to the style of cutting which consists of a table facet and a series of facets, each similarly disposed so that the contour of each runs parallel to that of the table. Also known as step cut. See Baguette-cut and Cuts of stones, Charts 181, 185.

Trapeze the name applied to a stone cut in the form of a trapezoid (two parallel sides and two inclined). See Cuts of stones, Chart 182.

Trapiche emerald an emerald crystal in which radially arranged albite separates clear emerald segments but producing a single crystal. The clear emerald parts are cut as gemstones.

Treated opal black dyed opal.

Tremolite Monoclinic; R.I. mean 1-61; S.G. 2-98; H. 5.5 to 6.5; An end member of the amphibole series of minerals, the other end member being actinolite (which see). The pale colours of nephrite are felted fibres of tremolite. The mineral also produces greenish chatoyant stones; (Canada), and a pink variety, called hexagonite (U.S.A.).

Triangle the name applied to a trap-cut stone with an outline in the form of a triangle, generally an equilateral triangle. See Cuts of stones, Chart 182.

Triboluminescence the phenomenon exhibited by certain minerals when they are rubbed or scratched, of showing a luminosity.

Triclinic one of the crystal systems. See Chart 1 11.

Trigonal or Rhombohedral system sub-division of the hexagonal system of crystallisation in which the principal axis is one of threefold instead of sixfold symmetry. Considered by some authorities as a separate system.

Trimetric system alternative name for the rhombic system.

Triplet a composite stone. See Charts.

Triplex opals a composite stone consisting of an opal doublet (which see) which has a covering dome of rock crystal cemented over the face of the opal. It is actually a triplet.

Tripoli a very fine-grained silica from Missouri and Oklahoma used as a polishing agent.

Tugtupite a cyclamen-red ornamental mineral found in Greenland. It is near sodalite in composition and has a density near 2-36 and a refractive index of 1-50. The hardness is 6.

"Tully" Memorial Medal; a medal awarded annually to the candidate submitting the best papers in the Pellowship Examination of the Gemmological Association of Great Britain which, in the opinion of the Council and Examiners, are of sufficiently high standard to merit the award.

Tumbling a method used for the production of baroque shaped stones by churning them with abrasive and then with polishing compounds in a rotating drum. Used considerably in America.

Turpentine an oil having a value in certain refractive index tests. R.I. = 1 47.

Turquoise a basic phosphate of aluminium and some copper and iron; R.I. 1 61 to 1 65; S.G. 2 6 to 2 85; H. 6; Triclinic; Colours, blue and green; Localities, Persia, Egypt, Turkestan and U.S.A.

Turquoise matrix Turquoise cut with some of the matrix it is found with, a brown limonite.

Turquoise, Viennese see Viennese turquoise.

Twin crystals two or more crystals of the same species which have intergrown together but always with reference to definite laws. They are often characterised by having reentrant angles, and are of three general types:

(a) Contact twins; where two halves of a crystal are in reverse order, so that if one half is rotated through half a circle about the plane of joining (perpendicular to the twinning axis) the form of the normal crystal is obtained.

(b) Interpenetrant twins; where two crystals have grown so that they penetrate one another, often producing cross and star forms.

(c) Polysynthetic or repeated twins; are composed of a number of contact twins producing very thin plates, each crystal being arranged in reverse order to its neighbour. Sometimes called lamellar twinning.

Two-point see Diamond point.

Ugrandite series the name applied to the isomorphous series of garnets which contain the uvarovite/grossularite/andradite group.

Ulexite (Mean) R.I. 1-51; S.G. 1-65 to 2-0; H. 1 to 2; Monoclinic (fibrous massive); Not a gemstone but of interest in that the fibres are so straight that when cut as thick plates with the fibres at right angles to the cut, print can be read through the stone and it is to this effect that the stone is called "Television stone".

Ultra-violet light the invisible rays, in wave-length beyond the visible violet, having a range of between 1,000 and 3,800 Angstrom units. They are known best by the facility with which they affect a photographic plate and by the effect certain substances have of emitting a visible light when they are irradiated with ultra-violet rays (Fluorescence). Between 3,800 and 3,000 A the rays are usually known as long-wave ultra-violet light, and between 3,000 and 2,000 A as shortwave rays. Below about 2,000 A the radiations, for physical reasons, cannot be properly used.

Ultra-violet lamps lamps used to produce ultra-violet light particularly for use in exciting luminescence in materials. They may be one of three types:

(a) Electrically excited high pressure mercury vapour in a quartz glass tube with the powerful visible light filtered out with a suitable filter (Wood's glass filter). The main ultra-violet rays produced by such a lamp are about 3,650 A. This lamp is known as the long-wave lamp.

(b) Electrically excited low pressure mercury vapour in a quartz, or special glass, envelope. The weak visible light and the long-wave ultra-violet rays are as far as possible filtered out by a suitable filter. Major radiation produced by this lamp is in the short-wave region at 2,537 A.

(c) The fluorescent tube lamp. The lamp consists of a shortwave ultra-violet generator as in (b) but enclosed in an ordinary glass envelope upon the inside of which is sprayed a compound which "fluoresces" in the long-wave ultra-violet region. The lamp thus produces long-wave ultra-violet light with a range between 4,100A and 3,200 A so that the lamp emits further into the visible violet than does the high-pressure long-wave lamp. This lamp may be best for distinction between natural and synthetic emeralds.

Unakite a granite-like rock consisting of a mixture of pink feldspar and green epidote and some quartz. The R.I. varies according to the mineral part from 1 52 for the feldspar, 1 55 for the quartz and 1 76 for the epidote. S.G. varies from 2 88 to 3 2. Mottled pink and green. U.S.A.

Uniaxial the term used to describe the optical character of anisotropic crystals which have one direction of single refraction; confined to crystals of the tetragonal, trigonal and hexagonal crystal systems.

Unit cell the unit of crystal structure which is the smallest part of a crystal which can still possess the characteristic properties of the crystal.

Units of weights see Precious Mmetal or Gemstome Chart.

Univalve molluscs pearl producing shellfish which have a shell in one piece, and not a shell in two halves as in the oysters and mussels. Such univalves are the abalone or Haliotidae and the giant conch.

"Uralian emerald" a MISNOMER ( COMMERCIAL LABEL ) for the green demantoid garnet. See Garnet, Andradite.

Utahlite alternative name for variscite.

Uvarovite see Garnet.

Valency the combining power of an element; measured by the number of atoms of hydrogen with which it can combine or which it can replace. Thus chlorine combines with one atom of hydrogen and is, therefore, univalent, calcium replaces two atoms and is, therefore, divalent, and so on. Several elements have different valencies in different compounds, e.g., iron is divalent as in the compounds FeO, or trivalent as in the compound Fe2O3. The valencies of the commoner elements are given on Chart 209.

"Vallum diamond" a MISNOMER ( COMMERCIAL LABEL ) for rock crystal.

Variscite R.I. mean 1 57; S.G. 2 52 to 2 60; H. 4 to 5; Rhombic; Colours, apple-green, blue-green; Locality, U.S.A. Sometimes called Utahlite.

Vaseline a hydrocarbon jelly useful for protective covering of refractometer dense glasses when the instrument is not in use. At a pinch this jelly could be used as an immersion "fluid" for small peelings in refractive index tests. The R.I. is near 1 50.

Vegetable ivory see Ivory, vegetable.

Velocity of light see Speed of light.

Venus hair stone see Quartz.

Verd antique see Serpentine.

Verdite a massive deep green micaceous rock used as an ornamental stone; R.I. 1-58; S.G. 2-8 to 3-0; H. 3; South Africa and U.S.A.

Verneuil process the method devised by the French chemist Verneuil, for the production of synthetic corundum and spinel, by use of the oxy-hydrogen furnace.

Vesuvianite see Idocrase.

"Victoria-stone" an artificial chatoyant stone made in various colours. Made in Japan.

"Vienna turquoise" a MISNOMER ( COMMERCIAL LABEL ) for a glass imitation turquoise.

"Viennese turquoise" an artificial turquoise produced from precipitated aluminium phosphate coloured by copper oleate. The powder being consolidated by hydraulic pressure into solid form. The material has similar hardness, S.G. and R.J. to true turquoise, but does not decrepitate when heated with the blowpipe flame.

Violane massive violet-blue diopside, which see.

Volcanic glass see Moldavite, Obsidian and Tektite.

Vulcanite crude rubber treated with sulphur by heat; S.G. 1-15 to 1-20 (Ebonite 1-2 to 1-8).

Walker balance a balance on the principle of the steel-yard used for determining the specific gravity of fairly large mineral specimens. The specimen is suspended and moved along the beam until it counter-balances the constant weight. The reading on the graduated beam, at the point the specimen is suspended is recorded (reading A). The specimen is immersed in water and again counterpoised and the second reading taken (reading B). Formula: B divided by B minus A, gives the specific gravity.

Walrus ivory an ivory obtained from the canine teeth of the walrus (Odobaenus rosmarus).

Wardite R.I. (mean) 1-59; S.G. 2-81; H. 5; Light green or bluish-green with vitreous lustre. Occurs in nodules of variscite at Utah, U.S.A. Sousmansite may be identical with wardite.

Wart pearls another name for blister pearls, which see.

"Water chrysolite" a MISNOMER ( COMMERCIAL LABEL ) for Moldavite.

"Water sapphire" a MISNOMER ( COMMERCIAL LABEL ) for Iolite (Cordierite).

Wave-length the distance between the crests, or troughs, of two successive waves. In the case of the longer waves of the electro-magnetic spectrum, the wireless waves, they are measured in metres; while in the shorter waves of light, ultra-violet rays, X-rays, etc., they are measured in Angstrom units, which see. See also Electro-magnetic spectrum.

Westphal balance a type of steelyard balance fitted with a sinker at the end of the graduated arm. It is used for the determination of the relative density of heavy liquids. By the substitution of a suitable clip and pan in place of the sinker, the specific gravity of small gemstones may be determined. The clip is immersed in water and weights placed on the graduated arm until the beam is counterpoised (reading A), this is repeated with the stone in the pan (reading B), and then with the stone in the clip immersed in water (reading C). Formula B minus A divided by B minus C gives the specific gravity.

West's solution a highly refractive liquid (R.I. 2-059; consists of white phosphorus + sulphur + methylene iodide; in the proportions 8 :1 :1.

Whewellite R.I. 1-49 --- 1-65; S.G. 2-23; H. 2.5; Monoclinic; Colourless; Central Europe and France.

White gold gold alloyed with silver, nickel, platinum or palladium will produce a white alloy. More than 50 % of the first two metals are required in order to produce the white colour, hence the alloy is limited to 9 carat quality. 30% of platinum or 25 % of palladium is required to produce white gold of 18 carat quality. White gold is used to simulate platinum as a setting for precious stones.

White light light consisting of all colours (wave-lengths). Sometimes known as mixed light.

White opal opal showing flashes of colour against a whitish background.

Willemite R.I. 1 693-1 712; S.G. 3 89 to 4 18; H. 5 to 6; Trigonal; Colours, yellow, green, brown and reddish; Locality, U.S.A.

Williamsite see Serpentine.

Wiluite a name applied to the variety of idocrase found at the Wilui River, Siberia. See Idocrase.

Witherite R.I. 1-532 --- 1-680; S.G. 4-27 to 4-35; H. 3.5; Orthorhombic; White; England, Japan, U.S.A. and Canada.

Wollastonite R.I. 1 61-1 63; S.G. 2 8 to 2 9; H. 4 to 5; Monoclinic; White; Finland, Romania and Mexico.

Wonderstone see Nevada wonderstone and pyrophyllite.

Wood opal see Opal.

Wulfenite R.I. 2-30 --- 2-403; S.G. 6-7 to 7; H. 3; tetragonal; Orange, yellow, green, grey and white; Australia, Congo, Central Europe, Mexico and U.S.A.

Wyoming jade nephrite from Wyoming, U.S.A., but the name is sometimes used for jade matrix, which see.

Xalostocite is an ornamental stone consisting of crystals of pink grossular garnet in white marble. Also known as Rosolite and Landerite.

Xanthite name applied to a yellowish-brown variety of idocrase from New York State.

X-rays the electro-magnetic radiations discovered by Rontgen in 1895 (hence sometimes called Rontgen rays) and well known for their property of penetrating opaque substances to a varying amount. They have a very short wave-length (mean about one Angstrom), so short that Max von Laue, in 1912, experimented and found that the atomic structure of crystals acted as a diffraction grating for these rays and laid the foundation of crystal analysis by X-rays, work which has been followed up by Sir William Bragg and his son, Sir Lawrence Bragg. The X-ray method of pearl testing is based upon crystal diffraction, and depends upon the radial structure of the crystals of aragonite in true pearls as against the parallel arrangement of the crystals in the mother-ofpearl comprising the nucleus of cultured pearls. The spot diffraction photographs obtained by this method are termed "Lauegrams".

1 The differential penetrating power of the rays to certain substances is used, not only for assisting in identification of gemstones, but for the determination of pearls by revealing the fine structures of real pearls and the bead of cultured pearls. Both freshwater and cultured pearls fluoresce under X-rays and this gives some help in identification. Synthetic ruby phosphoresces under X-rays while the real ruby does not. This is a valuable aid in the case of awkward synthetic rubies.

X-rays, Transparency to a method of identifying gems.

Xylene a liquid hydrocarbon; R.I. = 1-49.

YAG (Yttrium aluminium garnet) an artificially produced stone made in various colours and marketed under the name "Diamonair".

Yellow gold the usual colour of gold. The alloying metals being silver and copper in the proportion of roughly three to two.

Yellow ground the name given to the top layers of the "blue ground" or kimberlite, filling the diamond pipes of South Africa, the yellow colour being due to the oxidation of the iron content.

Yellow pearls pearls of a yellow colour obtained from the Margaritifera carcharium found along the coast of Shark Bay, Western Australia.

Yttrium aluminium garnet (yttrium aluminate) a synthetically produced crystal with a garnet structure. It is produced in a number of colours and colourless. Known as YAG the stones are marketed as "Diamonair''. The density is 46, the hardness 8 and a refractive index 1 83.

Zerfass emerald synthetic emerald grown by the firm of Zerfass of Germany.

Zinc blende see Blende.

Zincite R.I. 2-013 --- 2-029; S.G. 5-43 to 5-7; H. 4 to 4.5; Hexagonal; Colour, red; Locality, New Jersey, U.S.A.

Zircon two distinct types:

1 Normal (High) type. R.I. 1-925 --- 1 983 to 1-933 --- 1-992; S.G. 4-60 to 4-70, usually 4-69; fully crystalline, tetragonal; Colours, honey yellow, light green, blue and red. The heat treated stones (colourless, golden-yellow and blue in colour) are of this type.

2 Metamict (Low) type. Composed of nearly amorphous Sio,2; and ZrO,2; due to "metamictic" decomposition of the "normal" zircon. R.I. 1-79 to 1-84; S.G. 3-95 to 4-10; Nearly singly refracting; Colours, leaf-green to dirty brownish green.

Between these two types there are found stones intermediate in character due to a partial decomposition of the "normal" type. They may display a marked banded or zonal structure. Either the "low" or "intermediate" types will return to the "normal" type on prolonged heating. Localities, Ceylon, Indo-China, Thailand (Siam), Brazil, Australia, Russia and France. Low and intermediate types are common from Ceylon. The hardness of normal zircon is 7 and of the low type 6.5.

Zonite jasper from Arizona.

Zoisite S.G. 3-35; H. Crystals 8.5:, massive material 6 to 6.5; R.I. 1-692 --- 1-700; Rhombic; Blue, and sometimes green, yellow, pink or brown, which, on heat treatment usually turn blue. Found in Tanzania and hence the stones are called tanzanite. A massive pink zoisite found in Norway is called thulite (S.G. 3-10), and a massive green variety containing large opaque ruby crystals is found in Tanzanla.