The crystals formed by proustite are prismatic, rhombohedral, and scalenohedral. This mineral also forms in massive or compact habits. It is a rich scarlet color and also has a scarlet streak, though it blackens on exposure to light. It is translucent to transparent.
The luster of proustite ranges from adamantine to submetallic.
FORMATION Forms in hydrothermal veins, where it is associated with other sulfosalts, including tetrahedrite and tennantite; with sulfides, such as galena; and with quartz.
TESTS Soluble in nitric acid. Fuses easily.
Group: SULFOSALTS
Composition: Ag₃AsS₃
Hardness: 2–2½
SG: 5.55–5.64
Cleavage: Distinct rhombohedral
Fracture: Conchoidal to uneven
