Proustite

proustite, a sulfosalt mineral, silver arsenic sulfide (Ag₃AsS₃), that is an important source of silver. Sometimes called ruby silver because of its scarlet-vermilion colour, it occurs in the upper portions of most silver veins, where it is less common than pyrargyrite. Large, magnificent crystals, of hexagonal symmetry, have been found at Chañarcillo, Chile; other notable localities are Lorrain, Ont., and Freiberg and Marienberg, Ger. It is mined as a silver ore in Mexico and is found in small amounts in the silver mines of the western United States. For detailed physical properties, see sulfosalt (table).