Dioptase, copper silicate

Dioptase is well suited as a pigment because of its not too high hardness. The complex copper silicate has a good light fastness, but discolors by heating - possibly even strong sunlight is enough. The mineral is very rare, especially the procurement of larger crystals for pigment production is difficult. Dioptase has a hardness of 5, which makes it suitable for use as a pigment.