Turquoise, a mineral valued as a semiprecious gem. It is the birthstone for the month of December. Turquoise is porous and opaque. It ranges in color from robin's-egg blue to gray-green and has a waxy luster. Stones sometimes called matrix turquoise or turquoise matrix have thin cobweblike veins of clay or iron oxide running through them. Turquoise is frequently used in the jewelry made by Indians in the southwestern United States, but it is used in other jewelry and ornaments as well.

The highest-quality turquoise is blue, although some blue stones fade and turn green with exposure to light and air. Faded stones and those of poor quality are sometimes dyed to improve their color. Bone turquoise, or odontolite, which is fossil bone rather than true turquoise, is also dyed to look like quality turquoise.

There are several chemical formulas for turquoise. It is primarily a hydrous phosphate of aluminum. The color is caused by traces of copper.

Turquoise has been mined in Iran and on the Sinai Peninsula since ancient times. In the United States it is found mainly in New Mexico, Arizona, Nevada, and Colorado.

Representative chemical formula of turquoise: CuAl6(PO4)4(OH)8 4H2O. Hardness: 6.5. Specific gravity: 2.6-2.9.