Proustite: Gemstone Information
Proustite is Ag3AsS3 and forms beautiful prismatic transparent adamantine dark red rhombohedral crystals of the trigonal system. Crystals are translucent, often darkening with exposure to light. The hardness is 2–2.5 and the SG 5.6–5.7. The RI for the ordinary and extraordinary rays is 3.08 and 2.79, uniaxial negative with birefringence 0.296. Proustite occurs as a late-forming mineral in hydrothermal deposits in the oxidized and enriched zone, with other silver minerals and sulphides. Exceptional crystals are found at the Dolores mine, Chanarcillo, Atacama, Chile. Fine faceted stones may turn out to be (admittedly rare) synthetic products.