Formula: Ag3AsS3

Colour: Scarlet, Vermilion or reddish gray

Lustre: Adamantine

Hardness: 2 – 2½

Specific Gravity: 5.57

Crystal System: Trigonal

Member of: Proustite Group, Proustite-Pyrargyrite Series. The As analogue of pyrargyrite. One of the so-called “ruby silvers”.

Name: Named by François S. Beudant in 1832 in honor of Joseph-Louis Proust (1754–1826), chemist and actor, for Proust’s work on the red silver minerals (proustite-pyrargyrite series). He is most famous for discovering the law of definite proportion, stating that chemical compounds always combine in constant proportions.

Type Locality: Jáchymov, Karlovy Vary District, Karlovy Vary Region – Czech Republic

Dimorph of: Xanthoconite