Chrysoberyl (alexandrite), quartz, garnet, tourmaline, peridot & precious metals

Alexandrite

Chrysoberyl

Tourmaline

Quartz - ametrine

Garnet - tsavorite

Peridot

Chrysoberyl

Chemistry: BeAl2O4 Color is yellow, green and brown. Luster is vitreous. Transparency: Crystals are transparent to translucent. Crystal System is orthorhombic Crystal Habits: Crystals tend to be blocky and are often distorted by twinning effects that sometimes produce a psuedo-hexagonal cyclic twin. also granular. Cleavage is fair in one direction and poor in another. Fracture is conchoidal. Hardness is 8.5 Specific Gravity is 3.7+ (above average for translucent minerals) Other Characteristics: Pleochroic (different colors seen from different viewing angles), high refractive index (around 1.75).

Gem varieties of chrysoberyl (three types)

The first type is simply faceted transparent chrysoberyl that is usually found in yellowish green to green, yellow and shades of brown. It is a fine gemstone, but is over-shadowed by its two cousins. The second variety is the "cat's eye", also known as cymophane. The effect is caused by microscopic needle-like inclusions that reflect light into a single dynamic sliver of light running along the center of the crystal The third and perhaps most interesting is Alexandrite. This rare and valuable gemstone has the unique property of changing color depending on the type of light that hits it. In sunlight, it appears almost emerald green, while in artificial incandescent light it appears a violet-red.

Geological setting of chrysoberyl

Chrysoberyl is a distant cousin to an emerald, hence the name chrysoberyl and beryl. The host rock for chrysoberyl is granite pegmatite, metamorphic schists, and secondary placer deposits. The Ural Mountain source is gone today, but other locales include Brazil, Sri Lanka, Zimbabwe, Myanmar, India, Madagascar, Tanzania, Tasmania, China, and in the US. This gem is very rare as the Russian mines were worked out by the 1900s. The only currently mined source of any quality alexandrite is from Brazil, where mining techniques are crude and 99% of the stones collected are rejected Historically, the largest alexandrite was found in Sri Lanka, at 1,876 carats; while the largest cut stone is 66 carats, on display at the Smithsonian Museum. The flawless Hope chrysoberyl is a light green faceted stone of 45 carats on display in London.

Alexandrite

This rare gemstone is named after the Russian tsar Alexander II (1818-1881), the very first crystals having been discovered in April 1834 in the emerald mines near the Tokovaya River in the Urals.

Alexandrite

Alexandrite is unusual because it is a grass green in daylight and raspberry red in artificial, incandescent lighting. The trace element Cr is responsible for both the red and green of alexandrite. The Cr in alexandrite is such that the color changes with wavelengths of light, from natural sunlight or fluorescent lighting, where is appears green, to indoor incandescent light, where is appears red

Geology of alexandrite

Metamorphic schists and pegmatites, just like other chrysoberyls

Synthetic alexandrite

Alexandrite is the birthstone for June, along with pearl. High quality, large alexandrite specimens are rare and expensive, and therefore this is a gem that has inspired imitations and synthetics. Alexandrite was successfully synthesized by 1973 and it is often imitated by synthetic corundum and synthetic spinel. The synthetic versions change from a gray/blue violet in daylight to a reddish-violet in incandescent lighting. See Chatham created alexandrite, which was first synthesized in 1975. Synthetics are Al2O3 with V

Shopping for alexandrite

Natural alexandrite will be very, very expensive in the finest qualities. Going as high as $50,000/ct. A nice greenish/yellow transparent chrysoberyl as shown above will generally sell for a few hundred dollars per carat. Chrysoberyl cat's eyes can range from a couple of hundred dollars a carat to tens of thousands per carat. Be careful with the jeweler that he is not selling a synthetic stone valued at only $10s/ct Chelsea filter view of synthetic alexandite

Quartz (SiO2)

It is the second most common mineral in the Earths crust The most diverse and abundant of all gem materials Hardness = 7, density = 2.7 g/cc, and R.I. = 1.55, hexagonal crystal shape

Varieties of quartz

Two general varieties: coarsely crystalline (rock crystal, amethyst, citrine, smoky quartz, rose quartz), and finely crystalline (chalcedony, agate, onyx, chrysoprase) The most valuable are the coarse varieties

Chrysoprase

Onyx

Amethyst crystals

Fine-grained varieties of quartz

Chalcedony is microcrystalline quartz, usually honey yellow, white Carnelian = red chalcedony (trace Fe) Chrysoprase = green chalcedony (trace Cu) Agate = variable colored chalcedony, often concentric bands chalcedony Onyx

Chalcedony

Carnelian

Chrysoprase

Amethyst

Shades of violet or blue; Siberian = dark purple, Rose of France = pale purple Amethyst may be found in alluvial deposits, lining volcanic cavities, or metamorphic rock fissures. Best amethyst comes from Brazil and Uruguay Means not drunken in Latin !!

Wanda amethyst mine, Argentina

Citrine

Derived from the French word citron meaning lemon Best citrine comes from Brazil, particularly the Sierra mine

Ametrine

Ametrine is a variety of quartz that contains both amethyst and citrine sectors in the same crystal. Best samples come from the Anah Mine, Bolivia Both amethyst and citrine are colored by small amounts of iron (approx. 40 parts per million). Amethyst color develops when iron-containing quartz is exposed to ionizing radiation. In nature, gamma rays from the decay of potassium-40 are the most likely source of ionizing radiation. The model currently accepted is that radiation oxidizes Fe3+ to Fe4+. Citrine color is from Fe3+.

Price and shopping for gem quartz

Ametrine, light colored amethyst (Rose du France) and citrine are only $1s/ct whereas deep colored amethyst goes for $10s/ct/ Be wary when buying from a store because they are often synthetic ametrine, citrine, and amethyst. Watch for absolutely even color and the absence of any imperfections. Can be heat treated quartz (nearly all citrine) or synthetic.

Garnet

Garnet is a group name that comprises minerals with common crystal structure but different chemical compositions. It has the composition X3Y2Si3O12 where X = Fe, Mg, Mn, and Ca, and Y = Al, Fe, and Cr The main species names are pyrope (Mg-Al), almandine (Fe-Al), spessartine (Mn-Al), grossular (Ca-Al), andradite (Ca-Fe), and uvarovite (Ca-Cr). The data common to all garnet is a hardness of 6.5-7.5, and a conchoidal to splintery fracture, brittle tenacity. Garnet's crystal system is isometric and shows dodecahedral form. It ranges from transparent to opaque, with a vitreous luster. Garnet has a refractive index of 1.72-1.756 and an S.G. = 3.32. Birth stone for January; and second wedding anniversaries

Geological setting

Garnets are mainly found in high pressure metamorphic rocks and in some volcanic rocks

Vapor-transported garnet rock

Eclogite

Mantle rocks (igneous)

Garnet schist

Color of garnet

Nearly all colors!! Tsavorite green Almandine burgundy red Pyrope - red-brown Spessartine orange-brown Andradite-orange brown (yellowish)

Worlds largest tsavorite

Asking price $2,200,000.for sale from Mike Couch in Des Moines

Varietal/trade names

Hessonite is a cinnamon orange type of grossular garnet Malay is a trade name for pyropealmandine-spessartine-grossular-andradite garnet with a range of colors from orangish brown to peachy pink. Rhodolite is a pinkish red garnet intermediate in composition between almandine and pyrope

Prices of garnet gems

Pyrope (red-brown) - $3-5/ct Rhodolite (pinkish red) - $10-30/ct Almandine (red-burgundy) - $3-5/ct Spessartine orange-red; moderate $2030/ct and expensive $50/ct+ Malay (yellow-green) - $25-50/ct Hessonite (yellow-brown) - $10-25/ct Tsavorite (green) - $40-100/ct

Other unusual garnets

Varieties of andradite are melanite (black), topaziolite (honey yellow), and demantoid (green). Demantoid has horsetail inclusions of fibrous minerals. They produce an interplay of light and are ~$1,000/ct for good ones!!

Tourmaline - borosilicate

(Na,Ca)(Li,Al,Mg,Fe,Mn)3(Al,Fe)6(B O3)3(Si5O18)(OH)4 Lots of varieties due to wide compositional variations and pleochroism H = 7, density = 3.0-3.2, hexagonal crystal system The name is derived from the Sinhalese word tourmali, which means mixed parcel. Birthstone for October

Tourmaline

Geological setting of tourmaline

Mainly pegmatites but also found in metamorphic rocks

Varieties of tourmaline

Rubellite = red to pinkred $20-100+/ct Verdilite = green $2050/ct Indicolite = blue green $50-$100s/ct Paraiba = neon blue green $1002/ct Dravite =brown and schorl = black are not sold as gems Watermelon = pinkwhite-green, $50$100s/ct

Synthetic tourmaline

Tourmaline is pyro- and piezo-electric, that is when rubbed or by heating and cooling and applying pressure, the crystal will become electrically charged. It was known in times past as the "ash puller," because it was used to pull ash from meerschaum tobacco pipes. This attraction for particles and dust means it must be cleaned more often than other gems. Synthetic tourmaline is used for industrial, not gem, purposes.

Peridot

Peridot (pronounced pear-a-doe) is the gem variety of the mineral olivine, and also called chrysolite. It is a magnesium iron silicate, (Mg, Fe)2SiO4, with a hardness of 6.5-7, and specific gravity of 3.28-3.48. Cleavage is indistinct and the fracture, conchoidal. Crystal form is rare, although it is in the orthorhombic crystal system. Peridot is the birthstone for August and 16th anniversary gem. It was probably called "emerald" in ancient accounts and Romans termed it "evening emerald."

Mogok, Myanmar

Geography, geology

Peridot was mined for over 3,500 years on the volcanic island of Zebirget, or St. Johns Island, in the Red Sea, and the deposits rediscovered in 1900. It is also found in Myanmar (Burma), Arizona, New Mexico, and Hawaii, in the US, China, Pakistan, Norway, Brazil, Australia, Kenya, Mexico, Sri Lanka, South Africa, and Tanzania. The United States was one of the largest producers of peridot, with an estimated value of production of $1.5 million in 1999. An estimated 80-95% of all world production comes from the Peridot Mesa on the San Carlos Apache Reservation in Arizona. Only the native Americans can mine it. In addition to being recovered from igneous basalt, gabbro, and peridotite, peridot also occurs in certain types of meteorites (but not gem quality!!).

Price and shopping for peridot

Depending on the intensity of the color, the price is $5-25/ct Very expensive for gems > 3cts There is a rather nasty peridot imitation stone being sold in some stores called synthetic forsterite. This is a very good imitation that is actually a type of synthetic peridot. So stay with a professional, independent jeweler when you shop for peridot. They will be the only people who can make sure you are getting the real thing. Strongly double refractive facet edges appear doubled as you look through the stone. If you look inside a peridot with a magnifying glass you will never be able to focus the back facet junctions. The stone splits the beam of light into two separate beams that cannot be focused.

Imitation peridot

Generally made from spinel (MgAl2O4)

Synthetic peridot

Precious metals (gold, silver, and platinum)

Precious metals (gold, silver, and platinum)

The precious metals are gold, platinum, and silver. They are all rare, with gold present in 3.5 parts per billion of the Earth's crust, platinum about 45 parts per billion, and silver in 73 parts per billion. These metals are strong and heavy because of their close-packed atomic structures and metallic bonding. Gold's specific gravity is 15-19.3 (density of 11 ounces per cubic inch). Silver's specific gravity is 10.5 (6.25 ounces per cubic inch). Platinum's specific gravity is 14-19. It does not rust or corrode and combines beauty with easy workability, rarity, and virtual indestructibility. It is malleable! Eighty percent of world gold production goes to gold jewelry

Gold jewelry

Pure gold is 24 karat (K). 12 k gold contains 50% gold. Lowest sold in the U.S. is 10 K. White gold is an alloy of gold and some white metals such as silver and palladium. Reddish gold is alloyed with Cu, greenish gold is allowed with Zn. The addition of another alloy makes the gold less soft (H = 2.3) White gold can be 18ct, 14ct, 9ct or any karat. New white gold rings are coated with rhodium to make them look whiter October, 2005: gold = $477/oz, silver = $7.80, platinum = $941/oz, rhodium = $2,930/oz December, 2007: gold = $793/oz, silver $14.11/oz, platinum = $1458/oz, rhodium = $6,588/oz

Geology of gold deposits

Biggest suppliers: South Africa, Canada, Australia, U.S.A. (Nevada), Russia Found as hydrothermal gold in veins or disseminations in various rock types. Placer gold is important Mined at grades as low 1-2 ppm (g/t)

Replica of Welcome Nugget

Cripple Creek

Silver

Silver is an element that occurs naturally in the Earth and in pure form, although it is too soft for practical use. Two-thirds of silver is found in association with copper, lead, and zinc, while the remainder is mined from goldsilver vein deposits. Silver is primarily used in the photographic industry and as digital cameras improve in quality and popularity, the silver industry as a whole will suffer. Biggest producers: Mexico, Peru, Australia, U.S.A. (ID, NV, AK). Silver is much less expensive than gold and platinum. The ease of working with silver makes it a favorite among jewelry designers. Sterling silver is (92.5% silver and 7.5% of another alloy, such as copper added for strength). Oldest mines are at Lavrion, Greece (4000BC)

Platinum

Platinum hammered strip was set into a gold and silver box from Thebes, fashioned in the Seventh Century B.C. and some flecks appear in gold Egyptian pieces from 1400 B.C. Some 2,000 years ago, South American Indians were the first to work and mine platinum, creating nose rings and jewelry items with a smelting process that would remain secret until the late 1700s. Platinum is an extremely rare substance, far less abundant than gold. It is found only in a few commercially viable deposits on the earth. Most platinum is mined in three major producing regions in South Africa, Russia, and North America. South Africa is by far the largest producer supplying about 2/3 of the total world production. Only operating Pt in the U.S.A. is the Stillwater mine, MT. Found in layered mafic intrusions (igneous rocks); Merensky Reef is the biggest (South Africa)