Turquoise

Turquoise, opaque, cryptocrystalline mineral, composed chiefly of hydrated aluminum phosphate, Al₂(OH)₃PO₄· H₂O, and prized throughout the world as a gemstone. It has a hardness of 5 to 6, a specific gravity of 2.6 to 2.83, and, in the unpolished state, shines with a feeble, waxy luster. The color ranges from blue and blue-green to greenish-gray, according to the various amounts of copper usually present. The mineral has been valued for its ornamental properties since ancient times and has been found in neckwear and bracelets recovered from old Egyptian tombs. The Aztec of Mexico commonly used turquoise for their fine mosaic art and introduced the stone to the surrounding areas, where it became known as chalchihuitl. It occurs mostly in the seams of igneous rocks, as incrustations on the surface of various slates, or as nodules in red sandstone.

The sky-blue variety of turquoise, commonly referred to as robin's egg, is the form most desired for jewelry. When excessively exposed to sunlight or heat, this variety may become dehydrated and turn green. Principal deposits of blue turquoise are located in Neyshābūr (Nishapur), Iran. Other varieties are found abundantly throughout Western Turkistan; Egypt, including the Sinai Peninsula; and Mexico. In the U.S., the mineral is worked extensively in the area of Santa Fe, New Mexico, and in parts of California, Nevada, and Arizona. See Gemstones.