turquoise, hydrous phosphate of aluminum and copper, Al2(OH)3PO4·H2O+Cu, used as a gem. It occurs rarely in crystal form, but is usually cryptocrystalline. Turquoise is opaque and has a waxy luster; the color varies from greenish gray to sky-blue. The sky-blue varieties are the most valued as gems, but because of their porosity they easily absorb dirt and grease and change in color to an unattractive green. Exposure to heat or sunlight is also injurious to the color of the turquoise. The finest specimens come from Iran; other sources are the Sinai peninsula and the SW United States, especially New Mexico, Nevada, Arizona, and Colorado. Turquoise matrix is a rock including fragments of turquoise, cut as a gem stone. Variscite, the hydrated phosphate of aluminum, is sometimes used as a substitute for turquoise. It occurs in crystals of the orthorhombic system and in massive form; minable deposits are found in Utah.