It crystallizes in the monoclinic system and is the stable form of silver sulfide below 173 °C (343 °F).

Acanthite is a common silver mineral in moderately low-temperature hydrothermal veins and in zones of supergene enrichment.

It occurs in association with native silver, pyrargyrite, proustite, polybasite, stephanite, aguilarite, galena, chalcopyrite, sphalerite, calcite and quartz.

[3] Acanthite was first described in 1855 for an occurrence in the Jáchymov (Joachimsthal) district, Ore Mountains, Bohemia (today Karlovy Vary Region, Czech Republic).

The name is from the Greek "akantha" meaning thorn or arrow, in reference to its crystal shape.