Dioptase is a stunningly beautiful gemstone. Originally mistaken for emerald due to its vivid green color, it is seldom used as a gemstone because, with a Mohs scale rating of 5, a specific gravity of 3.3, and perfect cleavage, it is much softer and more brittle than emerald. Usually only very small stones can be cut from this mineral.
Dioptase forms in parts of copper veins that have been oxidized by air or water and in some of the surrounding cavities. It is often associated with azurite, calcite, cerussite, chrysocolla, dolomite, limonite and wulfenite. Crystals of dioptase are transparent, and it is from this property that its English name is derived. It is taken from the Greek prefix dia, meaning “through,” and optazein, meaning “visible.”
Some specimens of dioptase are weakly dichroic. This means that they appear to change color, depending on the direction of the light source and the angle from which they are viewed. Although cutting may enhance this effect, it is rarely attempted because the mineral is fragile and the effort is unlikely to be rewarding. However, some specimens of dioptase are highly prized for their rich emerald-green color, sometimes with a bluish tint. When the color is particularly vivid, the stone becomes translucent rather than transparent.
Dioptase is one of the few silicate minerals that crystallize in the trigonal system of symmetry. The others include dolomite and willemite. As a result of this peculiarity, dioptase crystals may take on rhombohedral external forms. Their prismatic faces are sometimes so dark and reflective that the basic green color may appear cloudy and even black.
Historically, the most abundant source of dioptase was in Kazakhstan. Specimens mined in this country were wrongly identified as emeralds and sent to the Russian Tsar.
The main sources of dioptase are Chile, Democratic Republic of the Congo, Iran, Kazakhstan, Namibia, Russia, Aizona and California.