Dioptase
Though of a pretty, emerald-green colour, dioptase has never been found in large enough crystals for gem purposes, and it is, moreover, rather soft, the hardness being only 5 on Mohs’s scale, and has an easy cleavage. In composition it is a hydrous silicate of copper corresponding to the formula CuH2SiO4. The double refraction, which is large in amount, is uniaxial in character, and positive in sign, the ordinary refractive index being 1·667 and the extraordinary 1·723. Its colour and softness distinguish it from peridot or diopside, which have about the same refractivity. The name was assigned to the species by Haüy, from διὰ, through, and ὄπτομαι, see, because the cleavage directions were distinguishable by looking through the stone.
Dioptase has been found near Altyn-Tübe in the Kirghese Steppes, at Rezbánya in Hungary, and Copiapo in Chili, and at the mine Mindouli, near Comba, in the French Congo.