The opaque gems include representatives of metallic ores and rock-forming minerals, and with the exception of jade they are used mainly as ornamental stones. Some opaque gems such as obsidian, a volcanic glass, and jet, an extremely hard form of coal, are better classified as rocks. (An unusual mineral that rightfully belongs to this group is the pearl because, as with all gems, it is highly prized for its beauty.)
Turquoise is a sky blue gemstone that has been used as an ornament since the dawn of civilization. Jewelry made from turquoise has been found in Egyptian and Sumerian tombs, dating back to the fourth millennium B.C. Because of its softness (slightly less than 6 on the Mohs’ hardness scale), turquoise was easily worked with the primitive tools available in ancient times. The demand for the gem in the United States has been high because of the popularity of turquoise jewelry made by Native Americans in the West. For ages, the gem has been admired by the Navajos, who mined it long before the arrival of the Europeans. Most deposits were in New Mexico, Arizona, Colorado, and Nevada. The mineral is associated with copper and occurs in nuggets or in veins. Sometimes tiny veins of clay and iron oxide crisscross the stone, giving it a much greater appeal.
Jade has been highly regarded since ancient times for its pleasant green color and versatility. It is a waxy or pearly mineral that is usually green but is also yellow, white, or pink. Unlike other gems, which are usually varieties of a single mineral, jade has complex mineralogical attributes. It occurs in two varieties that are of different chemical composition but are similar in appearance. Jadeite is a pyroxene, and the light, translucent emerald green form is considered a precious stone. It is regarded as the more valuable of the two jades because it has a richer appearance and possesses a greater variety of colors. Nephrite is an amphibole and the more common of the two jades. Both varieties of jade have been carved into ornaments and implements since antiquity.
Moonstone, which is a variety of plagioclase called albite, is valued as a gemstone because of its bluish white or pearly opalescence. It receives its name from a moonlike silvery white sheen that changes on the surface as the light changes. Almost all moonstone of commercial value is from mines in Sri Lanka (Ceylon) off the southern tip of India. One belief is that knowledge of the future can be obtained by holding a moonstone in the mouth under a waning moon.
Malachite is a common ore of copper, and because of its conspicuous bright green color, it is a useful guide in copper prospecting. It is often found together with azurite, which forms deep blue crystals. Both occur in smooth or irregular masses in the upper levels of mines. Malachite sometimes possesses crystals with a glassy luster but usually occurs in fibrous rounded masses with a silky luster. The compact, deep-colored stones make beautiful ornaments when cut and polished. Malachite is also fashioned into urns, bowls, and a great variety of art objects. In the Middle Ages, malachite was especially treasured as a protection against the "evil eye."
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