EOSC FINAL EXAM - UNIT B

Summarize the historical sources of diamond prior to 1867

Alluvial (panning) deposits in India

Discuss the significance of the discovery of diamonds in South Africa in 1867

Discovery of primary source rock (kimberlite) instead of just secondary (alluvial) deposits -> new demand for diamonds developed so it was not just for royalty and uber wealthy anymore

Summarize how the dominance of the De Beers group has changed in the last century

De Beers controlled South African mining, then other mines came around

Russian Mir Mine*, Aussie Argyle Mine, Diavik and Ekali mines in Canada. Now De Beers only has 7 major mines of 20 worldwide

Use the diagnostic properties of diamond (such as thermal conductivity, composition, crystal structure, primary morphology, etc.) to identify it.

Very high thermal conductivity, octahedral crystal habit, very hard (hardest material on earth), fluorescent

Describe the cleavage of diamond and its importance for diamond cutters

4-directional perfect cleavage so it's great for diamond cutters

Differentiate between Type Ia, Ib, IIa, IIb diamonds in terms of composition and colour

Ia - aggregated N impurities (98% of diamonds). Yellow hue due to blue light absorption

Ib - isolated single N impurities

IIa - flawless. no N or B impurities. (2% of diamonds). Clear white color

IIb - B impurities - blue/grey hue as all other colours except blue are absorbed. Extremely rare.

Identify the cause(s) of different colours of diamond

D - flawless, no impurities

Blue/grey - boron impurities

Yellow - N impurities

Pink/Purple/Red - Deformed crystal structure

Green - natural irradiation (Dresden Green)

Black - graphite inclusions

Identify common diamond treatments and methods of creating synthetic diamonds

Colour treatments - irradiation and temp/pressure annealing

Clarity treatment - laser drilling, acid boiling, glass filling

Synthetics - CVD ("3D printer") and high pressure high temp

Describe the geological conditions necessary for diamond formation and how diamonds are brought from the mantle to the surface

Formed min. 140km below the Earth's surface. But it is typically too hot (1500C) so need a cool pocket of 950-1400* to form at the right depth, so it is enough pressure but cool enough. Otherwise they will be melted.

Brought to surface by magma coming up the carrot-shaped kimberlite pipe, erupting in a specific type of volcano. As it shoots up the pipe, the magma picks up the diamonds and deposits them at the surface.

Explain why diamonds are rare

Formed deep below the Earth's surface and very specific geological conditions required to bring them up to the surface

Describe the global distribution of diamond deposits, geographically and geologically

Kimberlite pipes only found in old/cold archaen-age cratonic basement rocks. Canadian Shield, Siberia, Africa, Greenland, etc.

Briefly summarize the change in diamond production by country through history

1870 - 1930: primarily South Africa

1930-1960: other African nations

1960: Russia becomes dominant

1980: Australia

1990: Canada

Today: Australia, Botswana, Canada top 3

Define and describe the 4(+1) C's and how each affects the value of a diamond

Colour - DEF considered colourless/ J-Z is yellow

Cut - Ideal/Shallow/Deep (enhance brilliance)

Clarity - IF, VVS, VS, SI, I

Carat - larger size increases price exponentially

5th C - country of origin ( ethical)

Describe trends in diamond pricing based on differing clarity, colour, and carat size.

Most expensive diamond: huge size, D colour, IF, Ideal cut. non-linear relationship (i.e. 2 carat stone more than twice the price of 1 carat stone or larger difference between D and F if IF vs if SI)

Describe the Kimberley Process and comment on why it is effective in some countries and not in others

"to address the development, implementation, and oversight of a tracking system for the export and import of rough diamonds to prevent the exploitation of diamonds for illicit purposes such as war and inhumane acts"

Effective in law-abiding countries but not in war zones or underdeveloped countries w small-scale mines (the strength of the system depends on cooperation and countries taking it seriously) - Zimbabwe good example of it failing

Explain why Canadian diamonds have higher value than diamonds from other countries

Most ethically-produced stones, strict standards and easily traceable

Describe the behaviour of light (electromagnetic radiation) as a wave

Sinusoidal wave - higher frequency means shorter wavelength

Define colour, reflection, refraction, refractive index, total internal reflection, critical angle, dispersion and pleochroism

Colour - visible spectrum of light radiation

Refraction - enters medium and changes speed, therefore changes direction

Refractive index - higher RI leads to higher refractive angle

Total internal reflection: light travels below the critical angle, so all the light is reflected internally (ideal cut diamond)

Dispersion - clear light exits as rainbow

Pleochrosim -> different colours at different angles

Explain the relationship between colour and wavelength of light

Short to long wavelength: purple, blue, green, yellow, orange, red

Use the subtractive colour theory to explain colour in minerals and gemstones

A blue colour happens because all other colours are absorbed (especially yellow, blue's opposite). A white colour happens when all are reflected, and black when all are absorbed.

Describe the effects of illumination type on colour

Different sources of light have different wavelengths/energy, therefore when passing thru the medium of a gemstone the colours will be different (sometimes)

Differentiate between isotropic and anisotropic minerals

Isotropic - only one refractive index (cubic structure -diamond and spinel)

Anisotropic - multiple refractive indices

Describe the three main mechanisms of colour generation for gems

Idiochromatic - self-coloured (peridot)

Alochromatic - coloured due to impurity (emerald)

Pseudochromatic- "falsely" colored from physical optics (diamond, calcite, zircon)

Describe the optical effects of asterism, chatoyancy, iridescence and labradorescence

Asterism - 6 pointed star (due to mineral inclusions)

Chatoyancy - cat eye effect

irisdescence- internal scattering of light, like glitter

Labradorescense-different width feldspar layers cause different colours to be displayed (looks like a van gogh painting)

Describe the 5 main groups of transparency

Transparent (can read thru it)

Semi-Transparent (blurred)

Translucent (light passes thru but can't see thru it)

Semi-translucent (light only passes thru if thin)

Opaque (no light passes thru)

Describe briefly the significance of emerald and aquamarine through history

Intensely vivid colours have been valued for millenia (especially in Greece and in Bible, Aztecs, India)

Aquamarine similar attention but not as famous or as coveted

Use the diagnostic properties of beryl (e.g., hardness, refractive index, SG, habit, cleavage, fluorescence) to identify it.

hard (7.5-8), parallel striations on the crysal and flat terminations (not a pointed end). Basal cleavage and conchoidal fracture

Compare the properties of beryl to other gems we have studied thus far.

Not as hard as diamond, usually blue coloured but can be any colour, hexagonal crystal structure, transparent

Describe the mineral formula and crystal structure of beryl

Be3-Al2-Si6-O18

hexagonal structure

substitutions to Al cause the vivid colours

Explain the differences between the different gem beryl varieties (e.g., beryl, emerald and aquamarine)

Beryl - colourless, opaque

Emerald - Cr and V substitutions of Al

Aquamarine - Fe for Al

Define the main variables used to value gem beryl

Intensity and saturation of colour, carats, type of colour (emerald vs red beryl)

Identify common treatments and imitations of gem beryl

oiling to fill cracks, heat treating for colour

synthetics: only emerald. Aquamarine is abundant so no need.

Emerald imitated by green glass, thing emerald wafer on colourless aquamarine, peridot

Describe the 4 (four) main geological settings/genetic models for gem beryl

Pegmatite - no emeralds

Magmatic ( in situ or hydrothermal)

Metamorphic - best emerald deposits

Secondary deposits - eluvial (weathering), colluvial (gravity), alluvial (water)

Explain why gem beryl is rare

Beryl is relatively rare because there is very little of the element beryllium in the upper continental crust and it concentrates only in specific rock types, such as granites and pegmatites. Furthermore, beryllium is not usually concentrated enough to facilitate the growth of larger crystals suitable for the gemstone industry.

Describe the global and local distribution of gem beryl

beryl found wherever pegmatites are - which is everywhere

best locations tho -> US, Brazil, Russia, Pakistan

Emeralds*** -> Colombia the best place

Some beryl in the yukon and winnipegosis area

Describe the basic relationship between corundum and ruby+sapphire

Corundum -> parent element

Fancy sapphire -> any corundum gem not blue or red

Sapphire -> blue corundum

Ruby -> red corundum

List the historically important ruby and sapphire occurrences

Best rubies -> Myanmar

Best sapphires -> Sri Lanka

Use the diagnostic properties of corundum (e.g., hardness, refractive index, SG, habit, cleavage, fluorescence) to identify it.

Very hard (9), very dense (SG 2.5), sometimes fluorescent, hexagonal crystal stucture

Compare the physical properties of corundum to other gems we have studied thus far.

very hard, hexgonal crystal habit, any colours, dense SG

Describe the mineral formula and crystal structure of corundum

Al2-O3, hexagonal crystal, octahedra sharing corners and sides

Explain the differences between the different gem corundum varieties (e.g., ruby, sapphire, fancy sapphire, corundum with asterism)

all substitutions for Al

Ruby - prescence of Cr

Sapphire - Fe and Ti

Others - varies (usually mix of the above and other impurities)

Asterism - star shape from titanium oxide inclusions

Describe the main variables used to value gem corundum

Colour, then clarity. Size and cut as always

Identify common treatments and imitations of gem corundum

99% of stones are heat treated to improve colour and clarity

Can also undergo diffusion treatment: where an element not associated with the crystal is forced into the structure via heat, pressure, and chemical gradients.

synthetics easily produced

imitations: colourless beryl glued to coloured piece (doublet), spinel, garnet

Describe the 3 (three) main geological settings/genetic models for gem corundum

(1) primary metamorphic corundum in gneisses and marbles; (most common and best crystals)

(2) xenocrysts in alkali basalts; and

(3) secondary accumulation in placers

Explain why gem corundum is rare

corundum itself not super rare

• perfect trace element balance is needed

• require specific conditions

Describe the global and local distribution of gem corundum

Sri Lanka, Kashmir, Myanmar, Australia

Canada not much, but some sapphires on baffin island

Describe the term "pegmatite"

Best rock type for finding large gemstones

intrusive ingneous rocks very large in size, with large crystals

Describe the genesis of pegmatites using the terms parental pluton, fractionation, dikes, fertile or barren magma

Parental pluton - larger igenous rock body which the pegmatite came from.

Fractionation - sequential crystallization of the elements as the magma cools

Dikes are like veins of magma, forming pegmatites

Fertile magmas result in pegmatites, barren ones do not

Describe the role of gases and volatiles in the crystallization of pegmatites

Gases and volatiles act as a flux for pegmatites (they reduce the freezing point), which means the magma cools for longer in the liquid state, meaning more time for crystals to grow

Describe the general physical morphology of zoned gem-bearing pegmatites in terms of the location of gem-quality minerals

It has layers like an onion

core is made up of quartz

next layer is the core margin where the large gem quality crystals beryl/spodumene/tourmaline grow into fluid-filled pockets in the quartz core)

Explain why gem-bearing pegmatites are rare

Require granitic rocks and magma must have the chance to develop slowly.

The magmas must also be fertile, and there must be volatiles but not too many.

A “gentle” tectonic uplift, allowing pegmatites to rise to shallower crust.

AND there needs to be erosion so you can find the pegmatites.

Describe the global and local distribution of important gem-bearing pegmatite districts

Best region is in Brazil and in USA, but they are found everywhere.

Also Madagascar Russia Pakistan, Italy, Mozambique, Namibia, Afghanistan.

In Canada, best spot is Canadian Shield (superior province) - Manitoba

Use the diagnostic properties of tourmaline, topaz and spodumene (e.g., hardness, refractive index, SG, habit, cleavage, fluorescence) to distinguish them from each other and from other minerals.

Crystal habit and hardness the only way to differentiate. Topaz is hardest, then Tourmaline, then spodumene.

Tourmaline forms hexagonal crystals, topaz forms 8-sided prisms and spodumene forms rectacgular prisms

Compare the physical properties of tourmaline, topaz, and spodumene to other gems we have studied thus far.

All three are anisotropic (double refraction)

Tourmaline often has vertical striations along the crystal length

Topaz and Spodumene have perfect cleavages, but tourmaline has very poor cleavage.

Describe the mineral formulae and crystal structures of tourmaline, topaz and spodumene

Tourmaline: Borosilicate mineral group (Boron and Silicon).

Belongs to the hexagonal crystal system and forms pseudo-hexagonal prisms.

Base formula: XY₃Z₆(BO₃)₃Si₆O₁₈(OH)₄.

Common: NaFe₃Al₆(BO₃)₃Si₆O₁₈(OH)₄

Topaz: Aluminum fluorosilicate (Al, Si, F); belongs to the orthorhombic crystal system, forms 8-sided prisms.

Formula: Al₂SiO₄(F,OH)₂

Spodumene: Lithium aluminum silicate (Li, Al, Si); belongs to the monoclinic crystal system, forms rectangular prisms.
Formula: LiAlSi₂O₆

Explain the differences between the different topaz gem varieties (e.g. topaz and imperial topaz)

Topaz - colourless

Imperial topaz - orange-reddish

Colours generated by electrons in the holes of the crystal structure

Explain the differences between the different gem spodumene varieties (e.g. kunzite and hiddenite)

Kunzite - light pink (Manganese Mn3+ substitution)

Hiddenite - green (Chromium Cr3+ substitution)

Explain the differences amongst the tourmaline minerals and gem varieties (e.g. dravite, indicolite, rubellite, verdelite, paraiba, and watermelon)

Differences are the colours and inclusions. Watermelon tourmaline is caused by manganese interior and iron outside

look at chart

Describe why some gemstones are more commonly carved or cut 'en cabochon' rather than faceted

Either they are translucent or opaque and display better colour or texture when polished rather than faceted.

Describe the difference between nephrite and jadeite

Jadeite is comprised primarily of the mineral jadeite (NaAlSi2O6), but is still considered a rock because there are some other minerals contained in it as well. Nephrite is actually a mass of minerals in the amphibole group.

Jadeite is often described as having a granular texture and nephrite a silky texture, a direct result of their mineral compositions.

Use the diagnostic properties of jade, lapis lazuli, turquoise, amber, ammolite, pearl, and coral (e.g., composition, colour, unique physical and optical properties) to distinguish them from each other and from other gemstones and minerals.

Jade - Tough, green, translucent

Lapis Lazuli - blue colour from lazurite, pyrite flecks (golden) it is a rock

Turquoise - blue to green, but with Cu and opaque

Amber - transparent/translucent, lightweight, can contain insects.

Ammolite - fossilized rainbow shell of ammonite

Pearl - pearls formed from oysters

Coral - actual coral from the ocean

Describe how pearl, amber and coral are formed and how they differ from most other gemstones

• Pearl: Clam (Formed by mollusks (immune response to foreign material, aragonite layers).)

• Coral: Coral Reef (Skeletal material of coral polyps (CaCO₃).)

• Amber: Fossilized tree resin.:

• These gems originate from biological processes, not mineral crystallization.

Use the diagnostic properties of alexandrite (gem variety of chrysoberyl) and tanzanite (gem variety of zoisite) to distinguish them from each other and from other minerals.

Alexandrite - Colour changes from blueish-green (daylight) to purpleish-red. Caused by Chromium absoportion

Tanzanite - trichoic, deep blue/purple, larger stones, softer, more dense. Mined in Tanzania at base of mt kilimanjaro

look at chart

Differentiate between the UGRANDite and PYRALSPite garnet groups

Main Cation

Ugrandite - calcium. Green or yellow

Pyralspite - aluminum. Red, brown, pink

Recognize the gemstone peridot (gem variety of olivine) and the four main gem varieties of the quartz family (opal, amethyst, citrine, and agate) by using their distinguishing physical properties (e.g., colour, banding, optical effects, etc.).

Peridot: Bright green to yellow-green; birefringent doubling of facets; olivine​
Opal: Play-of-colour; amorphous SiO₂; soft (5.5–6.5)​
Amethyst: Purple quartz; Fe impurities; hardness 7​
Citrine: Yellow/orange quartz; Fe³⁺; transparent; hardness 7​
Agate: Banded cryptocrystalline quartz; hardness 7

How are gem corundum deposits in alkali basalts similar to diamond deposits?

Both are formed in the upper mantle and brought to the surface by magma of a different composition.

Would you be more likely to find a gem quality ruby in a metamorphic marble or xenocrystic alkali basalt deposit? Why?

Metamorphic marble Deposit.
Because its a primary deposit, primary deposits produce the best quality rubies and sapphires.

Primary deposits have the right chromium rich conditions while alkali basalt deposits are much rich in iron and titanium (better for sapphires).

Why might xenocrystic corundum be more common than xenocrystic diamond?

This involves the “window” of corundum, as it is more voluminous and survives better under more mantle conditions.

How might corrosion in a pegmatite affect a faceted gemstone, such as beryl or spodumene?

Alter the external shape (e.g., from sharp prismatic crystals to rounded or "bullet-shaped" crystals).

Cause internal flaws, cloudiness, or cracks, which can lower the clarity, durability, and value of a faceted gemstone.

What property can make topaz difficult to work with in jewellery?

Perfect cleavage, breaks easily

Why are jade and lapis lazuli considered rocks and not minerals like most gemstones?

Because both are mixtures of minerals