Dioptase

Dioptase is a hydrated copper silicate (CuSiO₂(OH)₂) that crystallizes in the trigonal system, forming characteristic prismatic crystals of extraordinarily vivid emerald green color. It forms exclusively in supergene environments, that is, in the surface alteration zones of copper deposits, where aqueous solutions rich in silicon dissolve primary copper minerals (chalcocite, chalcosite) and recrystallize them into more stable forms. Its moderate hardness (5 on the Mohs scale) makes it brittle for gemological use, but its transparency and saturated color make it extremely sought after by collectors. The most celebrated localities are Kazakhstan (Dzhezkazgan mine), Chile (Atacama), Namibia, and Peru, where crystals reach exceptional sizes. Its rarity and beauty position it among the most appreciated minerals by mineralogists, although it is practically absent from the commercial gemological market.

Dioptase crystallizes in the trigonal system (space group R3̄m), with lattice parameters a = 7.70 Å, c = 5.07 Å. Density is 3.28–3.35 g/cm³. The ordinary refractive index (nₒ) is approximately 1.644 and the extraordinary index (nₑ) is approximately 1.709, with positive birefringence of ~0.065. Hardness is 5 Mohs, with conchoidal fracture and imperfect cleavage along 1010. The intense color derives from Cu²⁺ in distorted octahedral coordination; the absorption spectrum shows characteristic bands in the green-blue region (around 650 nm and 450 nm). Chemical composition is stable only in neutral to alkaline pH environments and low oxidation potential; it frequently associates with limonite, malachite, azurite, and quartz. UV fluorescence is generally absent or very weak. Solubility in dilute acids is slow but appreciable, with release of Cu²⁺ ions and formation of silicic gel.