Dioptase is a mineral, copper silicate, crystallizing in the trigonal system. It forms short to long columnar crystals with a vault end, or occurs in granular to integral aggregates. Dioptase is an unusual mineral that is produced mainly in desert areas. It is very rare and occurs only in a few known sites in the world - in Congo, Namibia and Kazakhstan. Typical is its dark green color, for which it is very popular and sought after collectors. Interestingly, the dioptase crystals were used to adorn the Neolithic sculptures dating from around 7200 BC.