Turquoise is a blue hydrated basic copper aluminum phosphate mineral, CuAl₆(PO₄)₄(OH)₈.4H₂O; used as a semiprecious gemstone, found in aluminum-rich rocks in deserts. Deposited from water, it occurs in veinlets and as masses, and is a secondary mineral in assiciation with aluminum igneous or sedimentary rocks that have undergone considerable alteration. Its crystal system is triclinic; color varies from sky-blue to blue-green to a greenish gray. The finest turquoise comes from Iran. Hardness: 6; relative density 2.7.