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Dioptase Mineral Facts:

Chemical Formula: CuSiO₂(OH)₂
The mineral is a hydrated metasilicate.

Colors: Emerald-green or blackish green color, and a green streak.

Hardness: 5

Density: 3.2 to 3.35

Cleavage: Rhombohedral, perfect three directional cleavage.

Crystallography: Rhombohedral
Its crystals are columnar. Besides occurring as crystals the mineral is found also massive and in crystalline aggregates.

Luster:. Vitreous, transparent to translucent.

Optics: (Refractive Index)  w=1.6580,
e=1.7079.

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Composition, Structure and Associated Minerals:
Dioptase is a rare secondary mineral found mostly in desert regions where it forms in the oxidized zones of copper ore deposits. Because of the formation of acids during the oxidation of common sulfide copper minerals like chalcopyrite, dioptase is commonly associated with limestone that neutralizes the acids. This neutralization step is required to create conditions that are favorable to dioptase formation.

It is commonly associated with other secondary copper minerals, including chrysocolla, malachite, azurite and native copper. Other associated minerals include the primary vein materials like quartz and calcite.

Identification and Diagnostics
Before the blowpipe dioptase turns black and colors the flame green (test for copper). On charcoal it turns black in the oxidizing flame and red in the reducing flame without fusing. It is decomposed by acids with the production of gelatinous silica. Resembles chrysocolla in its chemical reactions, but differs from it in yielding gelatinous silica when dissolved in hydrochloric acid. It has been mistaken for emerald, but the extreme hardness difference between the two is an easy test.

Occurrence, Localities and Origins:
Dioptase has a fine emerald green color and is sometimes used as a gemstone. Some crystals are clear enough to be faceted, though because of its low hardness and rarity, it is an unusual collectors stone and not likely to be seen in any jewelry store. Fine crystals on matrix make for attractive pieces for mineral collectors and the material is prized for that as well. It is also a very minor copper ore in some copper deposits.

The mineral occurs in druses on quartz in clefts in limestone, and in gold bearing placers in the Altyn-Tiibe Mt. near the Altyn Ssu River, in Siberia; in crystals on wulfenite and calamine and embedded in clay near Rezbanya, Hungary; with quartz and chrysocolla in the Mindonli Mine, Congo; in copper mines at Capiapo, Chile; and in Peru.  The finest specimens on earth were found at the Tsumeb Mine in Tsumeb, Namibia. Dioptase specimens from Tsumeb are wonderfully lustrous and transparent, with its crystal often perched on an attractive snow-white carbonate matrix. In the deserts of the southwestern US, dioptase has been mined at the Bon Ton Mines, Graham Co., Arizona; and near Riverside, Pinal Co., in the same State. In the Bon Ton Mines it covers the walls of cavities in the ore, which consists of a mixture of limonite and copper oxides. A notable occurrence is the old Mammoth-Saint Anthony Mine near Mammoth, Arizona where small crystals that make fine specimens for collectors. In addition, many small, pale-green colored crystals of dioptase have been recovered from the Christmas Mine near Hayden, Arizona. 

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