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a mineral whose structural features correspond to the so-called cyclosilicate type. Its chemical composition is Cu6[Si6O18] 6H2O. Dioptase crystallizes in the trigonal system, forming acicular short-columnar small crystals, and accumulates in druses or granular aggregates of bright emerald green color. It is transparent and has an intense luster. The mineral has a hardness of 5.0-5.5 on the mineralogical scale and a density of 3,280-3,350 kg/m3. It is usually found in the oxidized zones of copper sulfide deposits along with malachite, calcite, and quartz and sometimes with hemimorphite and other minerals. Dioptase was discovered by Ashir Muhammad of Bukhara in 1785 (deposit at Altyn-Tiube in Central Kazakhstan). There are also a number of deposits in Africa (Katanga and elsewhere).