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Minerals A-E
Dioptase
Name
Dioptase
Chemistry
CuSiO3 -H2O, Hydrated copper silicate
Uses
As a mineral specimen
Color
Emerald green, difficult to photograph with the digital camera.
Hardness
5-5.4
Specific gravity
3.3
Crystals
Trigonal.
Accompanied by:
Copper sulphide deposites.
Fracture
Conchoidal to uneven.
Luster
Vitreous.
Cleavage
Rhombohedral.
Streak
White
Similar to:
Emerald in color
Dioptase is one of the few silicates to crystallize in the same symmetry class as dolomite and forms crystals that can have a typical carbonates' rhombohedral shape. Dioptase shares this symmetry with the typically ice clear silicate phenakite and the fluorescent but rarely well crystallized Willemite.
Specimens of Dioptase are often deeply colored and show well developed crystals. The faces of the rhombohedrons, and even the prism faces, are very reflective due to a fairly high luster.
Crystals can be quite clear but at times they seem to be cloudy due to the deep color. The special attributes of a high quality specimen of dioptase are hard to describe adequately but must be observed in person in order to appreciate their value. Fine crystals of dioptase are still available in large supplies but their sources are few and the continued demand may put a squeeze on their abundance in the future.
Crystals are transparent to translucent.
The crystal habits include stubby crystals that are usually well formed prisms topped with the three faces of a rhombohedon. Prisms are six sided and usually not very long. Simple rhombohedrons are also common, at times a second rhombohedron will modify the primary rhombohedron faces producing a second set of three smaller faces. Crusts and massive forms are also seen. Cleavage is perfect in three directions forming rhombohedrons.
Dioptase is a very beautiful mineral and it is one of the few minerals that can challenge the peerlessness of emerald's deep green. Unfortunately it is rather soft (for a gemstone) and has good cleavage and therefore is not usually cut as a gemstone. The mineral specimens that dioptase produces, however, are truly a treasure for anyone that likes deep green colors.
Notable occurrences include Tsumeb, Namibia.
Dioptase at Tsumeb.
The dioptase from Tsumeb is the world's finest. Beautiful, lustrous, transparent, dark emerald-green crystals on white calcite crystals make spectacular specimens. Crystals have been found up to 5 cm in size, and crystals 1 cm in size are fairly common. An enormous amount of dioptase was recently encountered in the Fast 19 stope on the 32 level; the finest dioptase has come from the lower oxidation zone and has been collected during the last 5 years. A small find of unusual blue-green to turquoise-blue dioptase crystals has been reported later
The exceptionally beautiful crystals of dioptase at Tsumeb occur almost exlusively in the country rock, and are only rarely ssociated with other secondary minerals. I have noted the ssociation of dioptase. One finds dioptase most frequently with older dolomite and younger calcite crystals. Sometimes a second generation of dioptase formed which is younger than the calcite. Duftite and plancheite are the next most common associates, and they are always older than dioptase.
The best cabinet specimen is in a private. This is a 5-inch mound of intergrown crystals to nearly 2 inches, implanted on a 6 x 12-inch slab of matrix coated ty a druse of white calcite. Two other specimens, much smaller are perhaps more perfect and desirable, came from the same pocket in 1975 ~on the 3300-foot level). Both of these have suberb, gemmy cerussite crystals implanted on the dioptase crysals, which in turn are implanted on white calcite crystals.
