Proustite

Formula: Ag₃AsS₃

Origin of name: for J. Proust, French chemist

Crystal system: hexagonal

Crystal class: 3m

Twinning: common on {10-14} trillings, also common on {10-11}, on {0001} or {01-12}

Unit cell: a = 10.96 Å, c = 8.61 Å

Colour: scarlet red to vermillion red

Diaphaneity: 

Luster: adamantine to submetallic

Habit: crystals prismatic, rhombohedral or scalenohedral, highly modified, massive, compact, disseminated

Hardness: 2 to 2.5

Specific gravity: 5.55 to 5.64

Cleavage: 1; {10-11} distinct

Tenacity: conchoidal to uneven, brittle

Streak: bright red

Synonyms/varieties: ruby silver

Comments: dimorphous with xanthoconite