dioptase

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dioptase A rare mineral, formula CuSiO2(OH)2; sp.gr.3.3; hardness 5; trigonal; emerald-green, transparent to translucent; vitreous lustre; crystals usually short, prismatic, and terminated by rhombohedra, but also occurs massive; cleavage perfect rhombohedral; occurs in the oxidized zone of copper-sulphide deposits.