TURQUOISE (from the Persian word "firuza"), callaite (EN: turquoise, callaite, agaphite; DE: Turkis, Kallait; FR: turquoise, agaphite; ES: turquesa; RU: бирюза), is the mineral of the class of phosphates, CuAl6[PO4]4(OH)8 • 5H2O.

The composition of turquoise is not constant; there exists the continuous series between the aluminium end-member of turquoise and its pure ferruginous analogue, namely, the green chalcosiderite. The usual content of FeO is 0.5-0.9%, up to 10-20% within the strongly altered samples; the content of Fe2O3 is up to 3.5%. The impurities are: Si, Ca, Sr, in lesser quantities Zn, Mo, Ni, Co, V, Ti, Ba, Be, Mg, and others (partly within the mechanical inclusions). Turquoise crystallizes itself into the triclinic crystal system. The crystal structure is represented by the framework, which is made of the PO4 tetrahedra and Al(OH)6 octahedra, within the voids of which there are situated the Cu ions. The crystals are very rare; there are common the crypto-crystalline, partly bud-shaped aggregates, nodules, venulous isolates. The colouration of turquoise, namely, from the sky-blue one to the apple-green one, and grayish-green one, is caused by the content of copper; the yellowish and brownish colouration is associated with the isomorphous substitution of copper by iron. The impurities of halloysite and kaolinite form the whitish spots. The lustre is vitreous (up to the waxy one for the dense turquoise), weak. The hardness is 5-6. The density is 2500-2850 kilograms per cubic metre (at least 2750 for the fresh dense varieties). The reticular, or arachnoid, turquoise (Auminza, within the Uzbek SSR; Arizona state, within the USA) is represented by the isolates of the bright pale-blue colour, which are broken by the hair-shaped cracks, which are filled with the carbonaceous substance. The formations, within which the dark-coloured country rocks are riddled by the venules of turquoise, are known under the name of the "turquoise matrix".

Turquoise is the hypergene mineral, and forms itself mainly within the regions with arid climate. The main deposits (Biryuzakan, within the Northern Tajikistan; Nishapur, within Iran; Villa Grove, Serrillos, and others within the USA) are confined to the linear crusts of weathering on the secondary quartzites with the copper-containing sulfides (chalcopyrite, cuprous pyrite) and apatite, which have emerged during the hydrothermal alteration of the acidic effusives of the trachyte-liparite-dacite association, and of the stocks of the granitoids, which are comagmatic with them. Less often, turquoise may be found within the crusts of weathering on the sandstones and shales, which are containing phosphorus and copper (Kyzyl-Kum, within the Uzbek SSR; Wadi Magara on the Sinai Peninsula, within Egypt), and also within the zones of oxidation of the sulphidic deposits of copper (Kalmakyr, within the Uzbek SSR; Teghut, within the Armenian SSR; Castle Dome, within the USA). The pale-blue turquoise is valued since the ancient times (especially at the East), and, according to the modern classification, belongs to the jewellery-grade precious stones of the IV order. The green varieties are used as the jewellery-ornamental stones. Turquoise is easily susceptible to beautification (colouring), but the bright artificial colouration fades with time. Turquoise is widely imitated (there is used, for example, the porcelain mass, which is coloured by the copper phosphate).