Colored Stones - 1-7

Define Chemical Composition

Kinds and relative quantities of atoms that make up a material.

Define Chemical Element

A substance that consists of atoms of only one kind.

Define Crystal Structure

Regular, repeating arrangement of atoms in a material.

Define Gem Species

A broad gem category based on chemical composition and crystal structure.

Define Gem Variety

A subcategory of species, based on color, transparency, or phenomenon.

Define Group

A family of gems from several closely related mineral species.

What gems come from pegmatites?

Tourmaline, kunzite, aquamarine, and pink/yellow beryl

What gems come from superheated water?

Amethyst, imperial topaz, and fine emeralds

What gems come from metamorphic rocks?

Ruby, emerald, alexandrite, tanzanite, and tsavorite

What gems come from sedimentary rocks?

Opal, turquoise, rhodochrosite, and malachite

Define Alluvial Deposit

A deposit where gems are eroded from their source rock, then transported away from the source and further concentrated.

Define Contact Metamorphism

Localized changes caused by an igneous intrusion that takes place where the magma meets the surrounding rock.

Define Eluvial Deposit

A deposit where gems are eroded from the source rock but remain in a place close to the surface.

Define Heavy Minerals

Minerals dense enough to become concentrated and separated from lighter ones by the action of surface water.

Define Hydrothermal Fluid

Hot, high-pressure solution that can dissolve, transport, and deposit minerals from one place to another.

Define Metamorphic Rock

A category of rocks that have been altered by heat and pressure.

Define Metasomatism

A type of metamorphism where chemical changes in the minerals and rocks result from the introduction of material from external sources, often as hydrothermal solutions.

Define the Orogenic Belt

A long, widespread area in the earth’s crust where tectonic events cause major structural changes, often forming mountain ranges.

Define Overburden

The material covering a gem deposit that must be removed before production can begin.

Define Pegmatite

An igneous rock typically formed from cooling, once-molten granite that follows fractures in its surrounding rock.

Define Pneumatolysis

Crystallization of minerals from a gas.

Define Xenocryst

A “foreign crystal” that forms in unrelated rocks and is brough to the surface as a passenger in magma.

What happens to limestone when heat and pressure are applied?

It is transformed into marble.

Define Cleavage

A smooth, flat break in a gemstone parallel to planes of atomic weakness, caused by weak or fewer bonds between atoms, or both.

Define Conchoidal Fracture

A curved and ridged fracture in a gemstone, extending from the surface inward.

Define Cryptocrystalline

An aggregate made up of individual crystals detectable only under very high magnification.

Define Durability

A gemstone’s ability to withstand wear, heat, and chemicals.

Define Fracture

Any break in a gem other than cleavage or parting.

Define Microcrystalline

An aggregate made up of individual crystals visible under magnification.

Define Parting

A flat break in a gemstone caused by concentrated included minerals parallel to a twinning plane.D

Define Stability

How well a gemstone resists light, heat, and chemicals

Define Toughness

How well a gemstone resists breaking and chipping.

Define Twinning

Change in a gem’s crystal direction during or after growth.

Define Twinning Plan

Location of a change in crystal growth direction.

Define Unit Cell

The smallest group of atoms with both the characteristic chemical composition and crystal structure of a mineral.

What are the three different ways a gem can break?

Cleavage, parting, and fracture

Which element causes red in ruby and green in emerald?

Chromium

Which gems are cryptocrystalline aggregates?

Chalcedony and turquoise

What gems have a cubic (isometric) system?

Diamond, spinel, garnet, and fluorine

What gems have a tetragonal system?

Zircon and scapolite

What gems have a hexagonal crystal system?

Apatite and beryl

What gems have a trigonal crystal system?

Corundum, quartz, and tourmaline

What gems have an orhorhombic crystal system?

Topaz, iolite, tanzanite (zoisite), chrysoberyl, and peridot

What gems have a monoclinic crystal system?

Kunzite and moonstone

What gems have a triclinic crystal system?

Amazonite and rhodonite

Define Contact Twins

Crystals that look like one half is a mirror image of the other.

Define Penetration Twins

Crystal where some look as if they’ve grown through each other.

Define Secondary Twins

Crystal caused by cooling temperatures or the pressure of metamorphism

Define Polysynthetic or Lamellar Twins

Crystals that start out untwined but become twinned when the rocks they formed in are altered by pressure during metamorphism.

Define Centipede Inclusions

Inclusions in an orthoclase moonstone that are caused by fine stress cracks parallel to cleavage directions

What trace elements cause blue sapphire?

Iron and titanium

Mohs Scale 10

Diamond

Mohs Scale 9

Corundum

Mohs Scale 8

Topaz

Mohs Scale 7

Quartz

Mohs Scale 6

Orthoclase Feldspar

Mohs Scale 5

Apatite

Mohs Scale 4

Fluorite

Mohs Scale 3

Calcite

Mohs Scale 2

Gypsum

Mohs Scale 1

Talc

Define Adularescence

The cloudy bluish white light in a moonstone, caused by scattering of light.

Define Allochromatic

A gem colored by trace elements in its crystal structure.

Define Aventurescence

A glittery effect caused by light reflecting from small, flat inclusions within a gemstone.

Define Charge Transfer

The movement of electrons back and forth between ions, caused by the selective absorption of light.

Define Chatoyancy

Bands of light in certain gems, caused by reflection of light from many parallel, needle-like inclusions or hollow tubes.

Define Color Center

A small defect int he crystal structure of a material that can absorb light and give rise to a color.

Define Diffraction

A special kind of interference phenomenon that produces patches of pure spectral colors.

Define Dispersion

The separation of white light into spectral colors.

Define Doubling

The appearance of double images of a gemstone’s facet junctions on the side opposite the viewer.

Define Fluorescence

Emission of visible light by a material when it’s stimulated by ultraviolet or X-ray radiation.

Define Idiochromatic

A gem colored by an element that is an essential part of its chemical composition.

Define Interference

Interaction between two light rays with the same wavelengths as they travel along the same path.

Define Intervalence Charge Transfer

A process where two transition elements with different valences exchange electrons to selectively absorb light.

Define Iridescence

A rainbow effect created when light is broken up into spectral hues by thin layers.

Define Labradorescence

A broad flash of color in labradorite feldspar that disappears when the gem is moved.

Define Optic Axis

Direction of single refraction in a doubly refractive gem.

Define Orient

Iridescence seen in some natural and cultured pearls and mother-of-pearl.

Define Phosphorescence

Continued emission of visible light after UV or X-ray stimulation stops.

Define Pleochroism

When a gem shows different bodycolors from different crystal directions.

Define Refraction

Change in speed and possible change in direction of light as it travels from one material to another.

Define Transition Elements

Elements that can selectively absorb some wavelengths of visible light and produce color in gems.