1. Understanding Economic Geology --Eamon McCarthy Earls, 2015
Gemstones
Understanding Economic Geology
--Eamon McCarthy Earls, 2015

2. What are gemstones?
Pieces of minerals and rock cut and polished to create jewelry
Usually semi-precious, rare material
Liquidity—in economics, how easily an asset changes hands
Cash and credit in your bank account have high liquidity
Paintings, gems and gold bars have lower liquidity but a lot of value
Gems & jewelry make great pieces of art that also hold a lot of value which can be very useful if you don’t trust cash

3. Gemstones
Identifying and working with gemstones usually falls to professional jewelers rather than geologists
Cutting & polishing makes these minerals very beautiful but destroys crystal patterns that can be used to identify them
Increasingly, many gems are synthetically produced in labs

4. Precious Gems
Four Gems—All 8-9 on Mohs Hardness Scale

5. Ruby
Variety of corundum with chromium impurities that lead to a bright red color
Al2O3:Cr
Crystals with fewer non- Cr impurities sell for more
Lack of rutile impurities is a tip off that the ruby may be synthetic

6. Ruby Cut & polished ruby— note dark dots are inclusions
Rutile inclusions indicate that the gem is genuine—Ex) rutile inclusions in Quartz
Cut & polished ruby— note dark dots are inclusions

7. Ruby Distribution
Historically, main source on Mogok River Valley in northern Myanmar (Burma)
Smaller deposits in India, Cambodia, Thailand & Brazil
Major deposits discovered in Greenland since 1966
Climate change & melting glaciers revealing more
Spinel, a red mineral, is commonly found in close proximity to ruby and often mistaken during mining
Ex) spinel at right

8. Emerald Variety of beryl Be3Al2(SiO3)6 Enriched in Cr & V
Hexagonal/Dihexagonal crystal
Colombia contributes 70-90% of all the world’s emeralds—so called Colombian Emeralds— source of funds for rebel groups
Value determined by number of inclusions—inclusions are very common
Historic mining in Egypt & Austria
Placer deposits in Zambia’s Kafubu River 2nd largest
IG Farben produces synthetic hydrothermal emeralds
Flux-growth synthetic emerald production ended due to 1989 San Francisco earthquake

9. Sapphire Fe, Ti, Cu, Cr, or Mg enriched end- member of corundum Al2O3
Trigonal crystals—conchoidal fracturing like quartz
3rd hardest substance after diamonds & moissonite
Widely used infrared optics as well as jewelry
Insulators in solid-state electronics
Almost all from alluvium—widely distributed mines: Afghanistan, Australia, Vietnam, Sri Lanka, etc.
Biggest deposits in Madagascar
Montana leads US production
Hot isostatic pressing for synthetic production
Alumina+oxyhydrogen flame— Verneuil process, 1902

10. Opal Hydrated amorphous silica mineraloid Not technically a mineral
Low-temp. deposition in cracks in rhyolite, limonite, basalt, sandstone & marl
97% from Australia— particularly South Australia
SiO2·nH2O
Wide range of colors— sub-vitreous sheen

11. Diamonds Isometric, octahedral crystals Pure C
Slight impurities lead to brown, transparent, yellow, green, blue & black endmembers
25-75% age of Earth
Silicon carbide & cubic zirconium are not diamonds but closely resemble them
Industrial & gem-grade diamonds priced up by DeBeers cartel
Almost all diamond trade & cutting in London, Antwerp & Tel Aviv

12. SEMI-PRECIOUS GEMS

13. Aquamarine Blue beryl Placers common in Sri Lanka
Fe2+ & Fe3+ generate hue
Artificially produced from pink/yellow beryl with radiation/gamma ray treatments
Discoveries in Idaho, Wyoming & Minas Gerais, Brazil
Largest aquamarine found in Brazil—other large varieties in New England pegmatites
Be3Al2(SiO3)6

14. Goshenite Beryl variety named after Goshen, MA Pure, colorless beryl
Range of colors—green, pink, blue, yellow
Low-value gemstone
Important source of beryllium

15. Red Beryl First discovered in Utah, 1904 Mn3+ gives red color
Very rare—only found in Utah & New Mexico
Topaz-rich rhyolites

16. Morganite Pink beryl Pala, CA & Madagascar
Rose of Maine—largest morganite discovered at Buckfield Mine, Maine, 1989
Mn2+ drives color

17. Amethyst Quartz variety SiO2—forms conchoidal fractures
Name derived from Greek word for intoxication— amethyst bowls believed to prevent against drunkenness
7 Mohs hardness scale, rhombohedral
Irradited Fe3+ induces color
Heating to citrine & ametrine—yellow/brown colors
Quartz doped with ferric material can be exposed to x-rays & gamma rays to make synthetic amethyst

18. Amber Technically not a material & organic
Widely considered to be gem material

19. Tanzanite Sorosilicate zoisite Discovered in 1967, Tanzania Trichroism
Blue, violet & burgundy
Orthorhombic—(Ca2Al3(SiO4)(Si2O7)O(OH)) + (Cr,Sr)
Named by Tiffany & Co.

20. Processing, Treatment & Valuation

21. Value National gem organizations Ex) Gemological Institute of America
Wide fluctuations in value in response to trends
Tanzanite varies, diamonds relatively stable
Rare minerals can have high value but only for a small group of collectors
10x magnification added to evaluate in 1950s
Unusual optics add value
Ex) color zoning & asteria star effects
Large discoveries can drive down price—amethyst ceased to be precious after large discoveries in Brazil, 19th century

22. Value
Expert professions include jewellers, diamantair’s and lapidary (gem-cutting)
Carl Faberge—Faberge jewel eggs produced in 19th century Russia
Four C’s—color, cut, clarity & carats
Carat—SI unit=200 mg, varied by country until early 1900s
Cut with faceting machines
Facets—flat, window-like faces cut on gem surface
Cabochons—dome shaped stones

23. Processing Heating improves clarity
Citrine (rose quartz) made by heating amethyst
Higher heat builds ametrine
Aquamarine heated to remove yellow colors
Diamonds treated with boracic acid to prevent surface burns in jewelry
Emeralds commonly fissured—disguised with wax