A mineral having composition Ag₃AsS₃ and crystallising in the hexagonal system. It occurs in prismatic crystals terminated by steep ditrigonal pyramids, but is more commonly massive or in disseminated grains. There is good rhombohedral cleavage. Hardness is 2–2.5 (Mohs scale) and specific gravity is 5.55. The lustre is adamantine and the colour ruby red. It is called light ruby silver in contrast to pyrargyrite, dark ruby silver. Proustite is less common than pyrargyrite but the two minerals are found together in silver veins. Noted localities are at Chañarcillo, Chile; Freiberg, Germany; Guanajuato, Mexico; and Cobalt, Ontario, Canada.