Turquoise is an opaque, blue-to-green mineral that is a hydrated phosphate of copper and aluminium, with the chemical formula CuAl6(PO4)4(OH)8·4H2O. It is rare and valuable in finer grades and has been prized as a gemstone and ornamental stone for thousands of years owing to its unique hue. In recent times, turquoise has been devalued, like most other opaque gems, by the introduction onto the market of treatments, imitations and synthetics. The gemstone has been known by many names. Pliny the Elder referred to the mineral as callais and the Aztecs knew it as chalchihuitl. The word turquoise dates to the 17th century and is derived from the French turquois for "Turkish" because the mineral was first brought to Europe through Turkey, from mines in the historical Khorasan Province of Persia.
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