gem silicates

Many gems and precious stones are silicates. These include andalusite, topaz, aquamarine, emerald, garnet, cordierite, peridot, tourmaline, tanzanite, jade, kunzite, hiddenite, lapis lazuli, serpentine, rhodonite, diopside, zircon, moonstone, amazonite, orthoclase, and labradorite. Gem quality andalusite, a silicate of aluminum (Al2SiO5), occurs as greenish or reddish pebbles in Minas Gerais, Brazil, and in Sri Lanka. The variety chiastolite (also called cross-stone, or macle), in cross section, shows a black cross on a grayish ground; polished cross sections of the mineral are sometimes worn as charms. Topaz, a transparent, vitreous silicate of aluminum containing either hydroxyl radicals or fluorine [Al2SiO4(F,OH)2], s commonly colorless or some shade of pale yellow to wine-yellow; pale blue and pale green also occur, but natural red stones are uncommon. Important sources of topaz are in Russia, Siberia, Brazil, Australia, and Mexico and in New Hampshire, Colorado, and Utah in the United States. Beryl, a silicate of beryllium and aluminum, Be3Al2Si6O18, occurring in hexagonal crystals that may be of enormous size and are usually white, yellow, green, blue, or colorless, is commonly used as a gemstone. Brazil is the major producer; others are Zimbabwe, South Africa, Namibia, and the United States. The most valued variety of beryl is emerald, whose deep green color is due to small quantities of chromium compounds. Good specimens are the most highly valued of gem stones, Similarly esteemed in antiquity, the stones were used for ornaments in early Egypt where some of the first emeralds were mined. The finest emeralds occur in South America in Colombia, where they have been mined for over 400 years. It was highly prized in pre-Columbian Mexico and Peru, where it was cut in intricate designs. India, Zimbabwe, and Australia are minor sources of the natural stones. Synthetic emeralds are also manufactured in Germany, France, and the United States. Aquamarine, a blue to sea green beryl, differs from emerald only in color, which comes from trace amounts of scandium oxide. Sources of the gems include Brazil, Siberia, Myanmar, Madagascar, and parts of the United States. Morganite, a form of beryl colored pink or rose-lilac by the presence of cesium, is often found with peach, orange, or pinkish yellow beryl in southern California and New England. Heliodor is the golden-yellow form of beryl. Garnet is a name applied to a group of isomorphic minerals crystallizing in the cubic system. The garnets are double silicates; one of the metallic elements is calcium, magnesium, ferrous iron, or manganese and the other aluminum, ferric iron, or chromium. Six varieties (of which there are also intermediate forms) are distinguished according to composition-grossularite (calcium-aluminum), pyrope (magnesium-aluminum), almandite (iron-aluminum), spessartite (manganese-aluminum), andradite (calcium-iron), and uvarovite (calcium-chromium). The last of these, an emerald-green variety from Russia and Finland, is rarely suitable for gem use. Grossular garnet occurs commonly in a red, green, yellow, or brown shade, depending on the impurities; if pure it would be colorless. The yellow and brown stones, found chiefly in Sri Lanka, are used as gems under the names essonite (or hessonite) and cinnamon stone. Grossularite is found also in the Transvaal, in Mexico, and in Oregon. Pyrope, the most popular variety of garnet, is ruby-red. Mined in Bohemia, South Africa, and Arizona, it is sold as Cape ruby and Arizona ruby. Rhodolite, a mixture of pyrope and almandite from North Carolina, is rose-red or purple. Deep red, transparent almandite, called carbuncle, was formerly a very popular gem. Mined chiefly in Brazil, India, and Sri Lanka, Australia and parts of the United States are also important sources. Spessartite is a brown to brownish-red garnet from Bavaria, Sri Lanka, and parts of the United States; it is seldom used for jewelry. Andradite is a very common variety of garnet, usually some shade of red, black, brown, yellow, or green. Gem varieties include topazolite, which is similar in color and transparency to topaz; demantoid, a green variety with a high dispersion and adamantine luster; and black melanite. Demantoid is found in the Urals, and the other andradites come chiefly from Europe and the United States. Synthetic garnets, which are high-melting silicates that have a role in the jewelry business, have a high refractive index and a hardness approaching that of diamond. Amazingly transparent in solid crystal form, they are being cut into imitation diamonds. Yttrium-iron garnets can be fabricated into special shapes for use as microwave filters in the communications industry; yttrium-aluminum garnets also are being produced at an increasing rate for use both in electronics and as gemstones. Cordierite, or iolite, is a silicate of aluminum and magnesium sometimes cut as a gem when clear. The stones from the gem gravels of Sri Lanka have been called water sapphires. Cordierite is sometimes called dichroite because of its marked pleochroism (different colored light is transmitted in different directions. Peridot is the transparent green gem form of the mineral olivine, a silicate of magnesium and iron (Mg,Fe)2SiO4, crystallizing in the orthorhombic system. Specimens of a yellow-green hue have been called chrysolite. Sources of gem-quality olivine are Jazirat Zabarjad (Saint Johns Island), Egypt, in the Red Sea (mentioned by Pliny in CE 70 in his Natural History (AD 70), the Mogok district of Myanmar, and Arizona. Tourmaline, a vitreous, complex borosilicate of aluminum and alkali, with iron, magnesium and other cations, occurs in prismatic crystals, commonly three-sided, six-sided, or nine-sided, and striated vertically. Different crystal forms are usually present at opposite ends of the vertical axis. Three types of tourmaline, distinguished by the predominance of certain elements, are usually recognized: iron tourmaline, called schorl, is black in color. Magnesium tourmaline, called dravite) is brown. Alkali tourmaline is called rubellite when pink; Brazilian emerald when green; indicolite, or Brazilian sapphire, when blue; and paraiba when blue-green to purple. Colorless varieties are called achroite. The variations in color are dependent on the variations in chemical composition. Two or more colors may occur in the same stone, the colors being arranged in zones or bands with sharp boundaries between them. Some crystals are pink at one end and green at the other; concentric color zoning may also occur. Some Brazilian stones have a red core with a green exterior, separated by a colorless band; some stones from California are green within and red outside. Sources of the gem include Elba, Madagascar, Myanmar, Sri Lanka, the Urals, Siberia, Brazil, and Maine, Connecticut, and California in the United States. Tanzanite, the precious variety of the mineral zoisite, a silicate of calcium and aluminum (Ca2Al3(SiO4)3OH), was discovered in 1967 in the Umba Valley near the Usambara Mountains in Tanzania. It occurs as orthorhombic crystals, which may be colorless, yellow-green, brown, or blue to violet when found, but when heated to 300-400C, many of them turn blue-violet, which is the preferred color for this gemstone. The hue is attributed to the presence of small amounts of vanadium. Jade is the common name for two minerals used as gems, long prized by the Chinese and Japanese as the most precious of all gems. The more rare variety of jade is jadeite, a sodium aluminum silicate, NaAl(SiO3)2, usually white or green in color; the green variety is the more valuable. The commoner and less costly variety of jade is nephrite, a calcium magnesium iron silicate of varying composition, often containing fluorine and hydroxyl, white to dark green in color. Colors such as red, green, and gray may occur owing to the presence of iron, chromium, or manganese impurities, respectively. The most highly prized variety is jadeite of an emerald-green hue. The fine luster of polished nephrite is oily rather than vitreous, while that of jadeite is the reverse. Some colors are also peculiar to one stone or the other; for example, the popular apple- and-emerald-green jewelry jades are invariably jadeite. There are also great variations of translucency in both minerals. Jade was extensively worked and used for jewelry, ornaments, small sculptures, and utilitarian objects by primitive people, especially in Mexico, Central America, Switzerland, France, Greece, Egypt, Asia Minor, and New Zealand. Jadeite is found around the city of Mogaung in northern Myanmar, in Japan, and in Guatemala; nephrite in New Zealand, Turkistan, Siberia, China, Silesia, Wyoming, California, and British Columbia. Lapis lazuli, a silicate of sodium and aluminum, containing sulfur, and mixed with other minerals, is usually found in masses. Deep blue, violet, or greenish blue in color , the stone is often flecked with yellow iron pyrites. Sources of supply are Afghanistan, Chile, Siberia, upper Myanmar, California, and Colorado The mineral serpentine, which takes a high polish and is sometimes used as an ornamental stone, encompasses any of a group of hydrous magnesium-rich silicate minerals. The composition of these common rock-forming minerals approximates Mg3Si2O5(OH)4. Named in allusion to its resemblance to a snake's skin, serpentine is usually grayish, white, or green but may be yellow (chrysotile) or green-blue (antigorite); the green color is due to iron replacing magnesium. Often translucent, it has a greasy or silky luster. Serpentine deposits are found in Canada, South Africa, and Vermont and Arizona in the United States. Kunzite and hiddenite are clear, precious varieties of spodumene, a silicate of lithium and aluminum (LiAlSi2O6). These are valued more by collectors and museums than by the public, because their color fades when exposed to sunlight. Hiddenite is emerald green and kunzite is pink or lilac. Rhodonite is a silicate of manganese that occurs as rounded crystals, masses, or grains in various manganese ores, often with rhodochrosite. Fine-grained rhodonite of clean, pink color is a desirable gem and ornamental stone. It is found in the URL Mountains of Russia, where it is mined for ornamental uses, and in Sweden, New South Wales, California, and New Jersey. Gem-quality diopside, a silicate of calcium and magnesium (CaMgSi2O6), is a strong, clear green. Zircon, a clear to golden gemstone, is a silicate of zirconium, also containing thorium and uranium). Adularia moonstone is a silicate of potassium and aluminum, as are the feldspar minerals amazonite (a green variety of microcline feldspar), and orthoclase. Albite moonstone and labradorite, another form of feldspar that exhibits a play of colors, are silicates of sodium and aluminum to silicates of calcium and aluminum.