Turquoise was one of the earliest gems to be used by humans for ornamentation. Color can range from bright blue to grayish green, with a waxy luster. Mined throughout the world, turquoise is often chalky and therefore stabilized in a hardening process.
Chemical composition: Cu(Al,Fe3+)6(PO4)4(OH)8 4H2O; Hardness: 5-6