Turquoise is an opaque, blue-to-green mineral that is a hydrous phosphate of copper and aluminium. As a secondary mineral, turquoise apparently forms by the action of percolating acidic aqueous solutions during the weathering and oxidation of pre-existing minerals. For example, the copper may come from primary copper sulfides such as chalcopyrite or from the secondary carbonates malachite or azurite; the aluminium may derive from feldspar; and the phosphorus from apatite.Turquoise was among the first gems to be mined, and while many historic sites have been depleted, some are still worked to this day.
