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. Like most other opaque gems, turquoise has been devalued by the introduction of treatments, imitations and synthetics into the market. The robin’s egg blue or sky blue color of the Persian turquoise mined near the modern city of Neyshabur in Iran has been used as a guiding reference for evaluating turquoise quality.The gemstone has been known by many names. Pliny the Elder referred to the mineral as callais (from Ancient Greek κάλαϊς) and the Aztecs knew it as chalchihuitl. The word turquoise dates to the 17th century and is derived from the French turquois meaning "Turkish" because the mineral was first brought to Europe through Turkey. However, according to Etymonline, the word dates to the 14th century with the form turkeis, meaning "Turkish", which was replaced with turqueise from French in the 1560s. According to the same source, the gemstone was first brought to Europe from Turkestan or another Turkish territory.

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