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DIOPTASE
The name of "copper emerald," by which this mineral is sometimes known, well indicates both its composition and appearance. No other mineral so closely imitates the emerald in color, although it differs in being slightly darker and less transparent. It possesses also a distinct rhombohedral cleavage, so prominent as to give the mineral its name, from two Greek words dia, through, and optomai, to see, because the cleavage can be seen on looking into a crystal. Dioptase is a hydrous silicate of copper, having the percentage composition: silica 38.2, cupric oxide 50.4, water 11.4. It crystallizes in the rhombohedral division of the hexagonal system, forming short prismatic crystals resembling superficially an isometric dodecahedron. Its chemical characters sufficiently distinguish it from emerald, as it gives before the blowpipe the green flame of copper, and gelatinizes with hydrochloric acid. Its chief defect as a gem consists in its lack of hardness, which is only 5. It is therefore rather easily scratched. Its specific gravity is somewhat high, 3.28-3.35. It has vitreous luster, and is brittle.
The best dioptase comes from the Kirghese Steppes of Siberia, where it occurs on the hill Altyn Tube, occupying seams in a compact limestone. The crystals here are so perfect that they can be worn uncut. Small crystals and rolled pieces are found in auriferous sands in various places in the Jeniseian Government in Russia, and fine crystals are reported from the Mindouli mine in the French Congo State. It has also been obtained at the copper mines of Clifton, Arizona, but the crystals are small, and of little value for gem purposes.
200
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