Late in the 18th century copper miners in the Ural Mountains in Kazakhstan thought they had discovered a huge emerald deposit in the Altyn-Tube mine in Karagandy. Samples of the lustrous emerald-green crystals were sent to Moscow where it was discovered that they were too soft -- only 5 on the Mohs hardness scale -- to be emerald, which has a hardness rating of 8.
Dioptase in Limestone Kazakhstan
In 1797 the distinguished French mineralogist Fr. René Just Haüy (1743-1822) determined that the enigmatic green mineral from Kazakhstan was new to science, and he named it dioptase, based on the mineral's two cleavage directions: Greek dia ("through") and optos ("visible"). Haüy went on to become professor of mineralogy at the National Museum of Natural History and founder of the Musée Minéralogique. The mineral hauyne was later named in his honor.
By chemical composition, dioptase is a hydrous copper silicate. It is a secondary mineral that precipitates from aqueous solution. The copper is derived through the chemical breakdown of primary copper and silica-bearing minerals in aerated groundwater. Its hardness of 5 puts it in the same class as apatite and turquoise. Dioptase has a density or specific gravity of 3.28 to 3.35, in the same range as peridot and tanzanite. Its refractive index is 1.644 to 1.709, similar to diopside.
Dioptase color ranges from emerald-green to bluish-green. It is typically transparent to translucent, with a vitreous to sub-adamantine luster. Deposits of dioptase are found in Chile, Kazakhstan, Namibia, Peru, Russia, Arizona in the USA, and the Democratic Republic of the Congo.
Dioptase is one of a small number of copper-bearing gemstones. Others include turquoise, chrysocolla, malachite, azurite, larimar and paraiba tourmaline. Due to its softness and its perfect cleavage, dioptase is only rarely set in jewelry. But faceted dioptase is popular with collectors for its rich color and excellent luster. The small and well-defined deep green rhombohedral crystals are also popular with mineral collectors.