GEMS-3
(Last updated 1/4/09)


Olivine is the name given to the entire series of mixed magnesium iron silicates of the formula (Mg,Fe)2SiO4 between the two end member minerals forsterite (all Mg) and fayalite (all Fe). Olivines (peridotites) contain the same chemical makeup as the molten magma in the Earth's mantle, and are thus the earth's most common rock by volume. Hawaii's famous green sand beaches are made up of olivine, indicating their volcanic origins. Olivine has also been found in many iron-nickel meteorites, often as significantly sized crystals, sometimes making up over 50% of the meteorite's volume; and olivine was also recently found by the Rover on Mars.

Olivine's gemstone variety, peridot, displays a distinctive greenish-yellow to yellow-green color; the best quality peridot usually has an iron content of ca 15% and includes nickel and chromium as trace elements. One of the most ancient of gems, many of Cleopatra's favorite "emeralds" were found to be peridot when subjected to modern analysis, and probably originated from a deposit on the Red Sea's volcanic island of Topazos (St. John), which has been mined for almost 4000 years (the island also gave us the name for the mineral topaz). Today most peridot comes from the San Carlos Apache Reservation in Arizona, as well as from deposits in China, Pakistan, Burma, and Korea.

A pair of 1ct 7x5mm oval Arizona peridots set in 14kt yellow gold studs (left), and a beautifully cut 8ct peridot from Pakistan (right)


Tourmaline, commonly used to indicate a family of gemstones, is actually the name of a group of isostructural complex aluminum borosilicate minerals with the general formula AX3Y6(BO3)3Si6O18(O,OH,F)4, where A is sodium or calcium, X can be aluminum, iron, lithium or magnesium, and Y is usually aluminum, but can also be chromium or iron. One mineral in the group, elbaite - Na(Li,Al)3Al6(BO3)3Si6O18(OH)4, provides the majority of gem quality stones, but the opaque black mineral schorl (A=Na, X=Fe, Y=Al), the most common tourmaline, is also used in jewelry, and translucent crystals of the related brown dravite (A=Na, X=Mg) and green uvite (A=Na,Ca, X=Mg) are sometimes faceted as well.

With a complex structure that can easily accomodate many different trace impurities, tourmaline comes in more colors than any other gemstone. Its typical thin elongated crystals are frequently color zoned, and can often have over a dozen different colors (or shades of the same color) along their length. Crystals can also be concentrically zoned, eg, a popular variety known as "watermelon" tourmaline has a pink center, a white ring around that, and green around the outside of that. Available in colors from white (colorless) to black and in shades from pastel to vivid (more than 100 separate hues have been identifed), a recent trend in the marketplace is to name tourmaline by a simple color designation, eg, yellow tourmaline. However, particularly desirable colors have historical names that are still in use. Vivid pink to red stones that retain their color in incandescent light are called rubellite (left; rich in Li, free of Mg and Fe), vivid blues indicolite (center), rich greens verdelite (or elbaite), vivid greens (with a very high Cr content) chrome tourmaline (right), and particularly vivid (described as "electric" or "neon") greenish blues (containing both copper and gold) "Paraiba", after their Brazilian state of origin (found only in 1987, Paraiba tourmaline is already one of the most sought after and expensive gemstones in the world).

Among the least popular tourmalines for jewelry are the rare colorless gems (called achroite) and the abundant yellows (left), although a particularly vivid canary yellow (containing traces of magnesium) discovered in 2000 in Malawi may change the popularity of the latter. In contrast, pinks (containing manganese, and with color centers from natural irradiation) such as this 6.0ct 14x10mm oval pink tourmaline (right) from the Pala mine north of San Diego, are among the most popular colors.

Tourmaline also produces various types of phenomenon stones, including cat's eyes and bi- and tri-coloreds such as this 1.2ct elongated emerald ("tourmaline") cut "watermelon" tourmaline (left; with green, colorless/white, and red segments); it is also often found as inclusions, as in this 10ct 16x12mm emerald cut tourmalinated quartz (right) containing needles of black schorl encased in rock crystal (right)


The mineral zirconium silicate - ZrSiO4 - is found worldwide, but Thailand, Cambodia, Sri Lanka, and Burma produce the majority of zircons (gem material) and the greatest variety of colors - usually brown, but also colorless, yellow (jargoon), orange, red (jacinth), green, and blue. Zircons always contain hafnium (1-4%), usually uranium (ca 300ppm - its slow decay to lead provides a way of using zircons to date rocks billions of years old) and thorium (ca 150 ppm) as well. The "self-irradiation" caused by the decay of these radioactive impurities results in damage to the gemstone's atomic lattice, producing red-brown to amber color centers and eventually altering the ordered crystal to an amorphous glass. Zircons thus affected are termed "metamict". Almost all zircons are heat-treated to remove the color centers produced by such radiation damage. Colorless zircons, which have little uranium, are NOT the same material as manmade cubic zirconia (CZ; a form of ZrO2), but are often used as a diamond simulant because they resemble diamonds in both luster and fire.

A 2.75ct 9x7mm oval honey zircon (left), a 2.1ct 8x6mm oval blue zircon (center), and a 3.3ct 10x8mm oval green zircon (right) from Sri Lanka (green is generally associated with metamict gems)


Spinel is the name given to a group of ca 20 oxide minerals that have the general formula AB2O4 (where A represents a divalent metal ion such as magnesium, iron, nickel, manganese, or zinc, and B represents trivalent metal ions such as aluminum, iron, chromium, or manganese) and very similar crystal structures (that have the same high symmetry as diamond).

The gemstone quality mineral of the spinel group, magnesium aluminum oxide - MgAl2O4, is also called spinel. Pure spinel is colorless, but chromium impurities can give it a red color that mimics ruby; in fact, many famous spinels (eg, the 170ct Black Prince's Ruby that forms the centerpiece of the royal crown of England and the 352ct Timur Ruby, also owned by Queen Elizabeth) were thought to be rubies until recent times. Though less common than red, spinel comes in other colors as well: pink, yellow to orange (rubicelle), violet (almandine), blue (cobalt spinel), green (chlorospinel, containing traces of iron and copper, and zinc-rich gahnite), brown (picotite, or chrome spinel), and black (pleonast). The best spinels still come from Burma (the historic source of spinels), but fine gemstones are also found in Sri Lanka and Tanzania.

A pair of 0.5ct 7x5mm oval red spinels (left), an 0.4ct pear shaped blue spinel (center), and a 1.75ct 8.5x6.5mm oval violet spinel (right)


Spodumene, or lithium aluminum silicate - LiAl(SiO3)2, is a colorless mineral that contains three fairly recently discovered gem varieties: triphane - colorless to yellow, kunzite - pink to violet, and hiddenite - an unusual green color that is unlike either peridot or emerald. Triphane is fairly common but seldom seen as a gemstone (except in collections), while true hiddenite, found only in Alexander County, North Carolina, and only in small sizes, is so rare that it, too, is seldom seen outside of collections.

On the other hand, a commercially significant deposit of kunzite was discovered in 1902 in the Pala region of California (in conjunction with the pink beryl morganite and pink tourmaline), and large deposits have since been found in many other parts of the world; today most kunzite is mined in Brazil, Afghanistan, and Madagascar. Although considered a real challenge to the gem cutter because of its multiple cleavage planes, tendency to splinter, and strong pleochroism - making proper orientation critical during faceting, kunzite's lovely lavender color - from the presence of manganese (and enhanced by natural or artificial irradiation to produce color centers) - has made it an attractive and desirable gemstone. However, because its color centers are metastable, kunzite should be protected from heat and from exposure to strong light, either of which can cause its color to fade.

A 7.6ct 14x13mm pudgy pear cut kunzite from Afghanistan


Zoisite, calcium aluminum silicate hydroxide - Ca2Al3(SiO4)3(OH), is a structurally complex green stone that looks similar to aventurine. Known for nearly two centuries, a pinkish variety called thulite is used for beads and cabochons, and a bright green chromium-containing variety from Tanzania that also contains red corundum (ruby) is used as an ornamental stone. However, in 1967 a transparent, highly pleochroic, deep blue gemstone was found in the Merelani foothills of Mt Kilimanjaro. Much to the surprise of both minerologists and gemologists, this new gemstone turned out to be a zoisite; named tanzanite (and initially marketed) by Tiffany's, tanzanite has since become one of the most popular (and expensive) colored gemstones in the world.

A 28x25mm oval ruby-in-zoisite cabochon (left), and a 1.15ct, 6.5mm round tanzanite (right)


Cordierite, magnesium aluminum silicate - Mg2Al4Si5O18, is normally gray, but can occur as a leek-green gemstone called praseolite, and turns a deep blue-violet as its ferrous ion content approaches 10%. Cordierite also produce a highly pleochroic, blue gemstone called iolite (or water sapphire or dichroite), or can display a reddish color when it contains inclusions of hematite. Gem grade iolite is found primarily in Sri Lanka, India, Madagascar and Burma.


Chrysoberyl, or beryllium aluminum oxide - BeAl2O4, is not a well-known gemstone mineral even though it can produce extremely brilliant and durable gems (its hardness is just a bit less than that of sapphire) and one of its varieties is among the most expensive gemstones in the world. Colorless when pure, transparent chrysoberyl most often appears yellow, yellowish green, or brown due to iron (Fe+3) impurities.

A 1.3ct 8x6mm oval chrysoberyl (left), and an 0.5ct 6x4 oval cat's eye (right; "cat's eye" alone always refers to a chrysoberyl; the names of other cat's eye gems must include the type designation as well, eg, opal cat's eye)

A 1.6ct 7x5mm oval alexandrite cat's eye from India (left), and a pair of 2.4ct 8x8mm heart shaped lab grown alexandrites (right)


Apatite, calcium chlorofluorohydroxyl phosphate - Ca5(Cl,F,OH)(PO4)3 (the mineral of teeth and bones), is actually three different minerals depending on the predominance of chlorine, fluorine, or hydroxyl; but as all three minerals are usually present in every specimen, they are usually considered as one due to the difficulty in distinguishing among them. Apatite is widely distributed in all rock types, usually as small disseminated grains or cryptocrystalline fragments. However, large well-formed crystals, typically green, but also yellow, blue, reddish-brown, and purple, have been found in certain contact metamorphic rocks and can be cut as gemstones. The softness of apatite once precluded its widespread use in jewelry, but the discovery of a neon blue-green variety in Brazil that mimics the color of Paraiba tourmaline has sparked renewed interest in apatite as a gemstone. Gem quality material is also found in Mexico, Canada, Germany, and Russia.

A pair of 0.80ct 7x5mm oval teal colored apatites


The mineral lazurite, sodium calcium aluminum silicate sulfur sulfate - (Na,Ca)8Al6Si6O24(S,SO4), can form deep blue crystals, but is more commonly found in massive form and combined with other minerals, usually pyrite and calcite, in a rock (mixture of minerals) called lapis lazuli. First mined 6000 years ago, the finest quality lapis - a translucent blue containing no calcite or pyrite that looks very much like Bakelite plastic - still comes from a remote mountain valley in Afghanistan, although lower quality lapis is also found in Chile, and to a lesser extent Siberia, Italy, and the US (in California and Colorado).


Other common gemstone minerals still not described are the cryptocrystalline jades - jadeite and nephrite - and turquoise, the organic gemstones pearl and amber, and some less common gemstone minerals such as andalusite, sphene, amblygonite, and kyanite. I hope to correct this problem in the not too distant future.

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