This is a very nice specimen of dioptase crystals from the Republic of Congo. The entire specimen is 2.18" long and the lustrous, rich green crystals get up to about .31" (8mm) long. They have a saturated, dark, emerald-green color with lustrous crystals, all atop a matrix of blue-green botryoidal plancheite. A small association of calcite can be seen somewhat protruding from between some dioptase crystals.
Dioptase is an uncommon mineral found mostly in desert regions where it forms as a secondary mineral in the oxidized zone of copper sulfide mineral deposits. It is popular with mineral collectors due to it's vibrant color and it is occasionally cut into small emerald-like gems.