Turquoise is a famous and valuable hydrous copper aluminum hydroxy-phosphate mineral, CuAl6(PO4)4(OH)8·5H2O. It is the definition of the color term "turquoise", but the mineral actually ranges from blue to bluish-green to green to greenish-gray. It has a nonmetallic luster (dull or waxy luster on massive, cryptocrystalline specimens & glassy luster on macrocrystalline samples), a pale greenish-blue streak, is moderately hard (H = 5 to 6), has two cleavage directions, and conchoidal fracture. It rarely forms decent crystals and typically occurs in massive form or as finely-crystalline crusts or veinlets in rocks.
Turquoise forms in the alteration zone of some "porphyry copper" deposits and can be found as a vein-filling mineral in some volcanic rocks and some phosphate-rich sedimentary rocks.
Turquoise from the Turquoise Chief Mine, St. Kevin-Independence, Lake County, Colorado, USA (CMS # CSHS.9932, Colorado School of Mines Geology Museum, Golden, Colorado, USA).
Turquoise from the Cripple Creek District, Teller County, Colorado, USA (ER # 2533, Ed Raines loan to the Colorado School of Mines Geology Museum, Golden, Colorado, USA).