Group: Silicates – cyclosilicates
Chemical composition: CuSiO2(OH2)
Colors: Emerald to blue-green
Principal Sources: Kazakhstan, Iran, Namibia, the Congo, Argentina, Chile, USA.
Special Notes: Dioptase is an uncommon mineral found mostly in desert regions where it forms as a secondary mineral in the oxidized zone of copper sulfide mineral deposits. However, the process of its formation is not simple, the oxidation of copper sulfides should be insufficient to crystallize dioptase as silica is normally minutely soluble in water except at highly alkaline pH. The oxidation of sulfides will generate highly acidic fluids rich in sulfuric acid that should suppress silica solubility. However, in dry climates and with enough time, especially in areas of a mineral deposit where acids are buffered by carbonate, minute quantities of silica may react with dissolved copper forming dioptase and chrysocolla. The 18th century copper miners that first discovered it thought they had found emeralds, but this mineral has a hardness almost half that of emeralds.