Dioptase is a copper-containing silicate, which is noted for its highly saturated green to bluish-green hues. While the mineral is very popular with mineral collectors, a relatively low hardness (5.0) and scarcity of large, clean crystals have limited its appeal as a gemstone.
When a large number of green crystals were first discovered within modern-day Kazakhstan at the end of the eighteenth century, it was initially believed that a new emerald deposit had been discovered. However, subsequent analysis in Moscow determined that the hardness of the crystals was far below that of beryl, and they were deemed to be a previously undiscovered mineral species. It was subsequently named dioptase in reference to its two highly visible cleavage directions, and additional sources have since been discovered in Namibia and the Republic of Congo.