GEMSTONESUnderstanding 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 GEMSFour 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

RUBYRutile 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