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

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

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

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

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

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

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

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

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

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

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

SEMI-PRECIOUS GEMS

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

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

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

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

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

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

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

Processing, Treatment & Valuation

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

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

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