noun

• An ore mineral of silver typically occurring as small, silver-grey, thorn-shaped or prismatic crystals, or in polycrystalline masses.

  Acanthite is silver sulphide, Ag₂S, and is stable only up to 170–180°C; above that temperature the stable polymorph is argentite. Crystal system: monoclinic, though naturally occurring crystals of acanthite are often paramorphs after argentite, which is cubic.