Twinned crystals of dioptase on calcite substrate. Dioptase is a copper cyclosilicate mineral, chemical formula CuSiO3.H2O, belonging to the trigonal crystal system. It is formed as a secondary mineral by oxidation in deposits rich in copper sulfides. It may occur in amorphous or sub crystalline form filling up veins in, or as a coating on its host-rock, but also occurs as well developed, void-filling crystals in a variety of green colors, ranging from a soft light green to a dark-bluish green. In this sample the crystals of dioptase, some twinned, are intergrown with clear, crystalline calcite. Sample (actual width 120 mm) is from the famous subsurface mines of Tsumeb, Namibia, now long closed.