other gem materials

Gems or gem materials that are neither carbonates, silicates, nor oxides include diamond, turquoise, pyrite, marcasite, ivory and amber. Diamond is a mineral composed of pure carbon. Colorless or pale blue stones are most valued, but these are rare. Most gem diamonds are tinged with yellow; those with a distinct color are called "fancy" diamonds. Red, blue, and green are most rare, and orange, pink, violet, yellow, and yellowish green more common. The earliest sources of gem diamonds were India and Borneo, where they were found in river alluvium. In the early 18th century, deposits similar to those in India were found in Brazil, mainly of carbonados, although they may have been known as early as 1670. In 1867, a stone found in South Africa and recognized as a diamond triggered a wild search for diamonds, both in river diggings and inland. In 1870-71, dry diggings, including most of the now-famous mines, were discovered. Other important diamond-producing countries include Australia (now the world's leading producer of natural diamonds), Russia, Brazil, Angola, Sierra Leone, Namibia, Ghana, Tanzania, Venezuela, and Canada. Pyrite, also called iron pyrite, or fool's gold (because its color may deceive the novice into thinking he has discovered a gold nugget) is a relatively common, naturally occurring iron disulfide mineral. Marcasite, which has the same chemical formula as pyrite (FeS2), crystallizes in the orthorhombic instead of the isometric system. Turquoise, an opaque, blue to greenish mineral with a faintly waxy luster, is a hydrated phosphate of copper and aluminum (Al2(OH)3PO49H2O+Cu). The sky-blue varieties are the most valued as gems. The finest specimens come from Iran; other sources are the Sinai peninsula and the southwestern United States, especially New Mexico, Nevada, Arizona, and Colorado. Ivory, a dense, opaque, white to yellow-white variety of dentin of which the tusk of the elephant is composed, is largely calcium phosphate in the form of oxyapatite and a small quantity of calcium carbonate, bound together by large amounts of dentine, a proteinaceous organic substance. (The teeth of the hippopotamus, walrus, narwhal, sperm whale, and some types of wild boar and warthog are recognized as ivory but have little commercial value because of their small size.) Amber is a (usually) noncrystalline fossil resin of varied composition that has achieved a stable state through loss of volatile constituents and chemical change after burial in the ground. Gem quality amber is usually transparent, but some varieties are cloudy. Often orange-yellow to red-orange, amber can vary in color from yellow to red to green and blue. The most famous source of the world's amber is the Baltic coast of Germany. Amber is also found off the coasts of Sicily and eastern England and in Myanmar. In the Western Hemisphere, rich deposits occur in the Dominican Republic, Mexico, and the state of New Jersey.