A mineral of composition CuAl₆(PO₄)₄(OH)₈ · 5H₂O in which considerable ferrous ion (Fe²⁺) may substitute for copper. Ferric ion (Fe³⁺) may also substitute for part or all of the aluminium (Al), forming a complete chemical series from turquoise to chalcosiderite (CuFe₆(PO₄)₄(OH)₈ · 5H₂O). Turquoise with a strong sky-blue or bluish-green to apple green colour is easily recognized, and such material is commonly used as a gem. Some variscite, of composition AlPO₄ · 2H₂O with minor chemical substitutions of Fe³⁺ and or chromium ion (Cr³⁺) for aluminium and with a soft, clear green color, may be marketed as green turquoise.