Acanthite is a form of silver sulfide with the chemical formula Ag2S. It was first described in 1855 for an occurrence in the Ore Mountains in the present-day Czech Republic. The name is derived from the Greek "akantha" meaning thorn or arrow, in reference to its crystal shape. Acanthite crystallizes in the monoclinic system and is the stable form of silver sulfide below 173 °C (343 °F). As argentite cools below that temperature, its cubic form is distorted to the monoclinic form of acanthite. This crystal of acanthite, with dimensions of 4.0 cm × 2.5 cm × 1.5 cm (1.57 in × 0.98 in × 0.59 in), was extracted from a mine in Imider, in the Jbel Saghro, a mountain range in Morocco. This photograph has been focus-stacked from 42 separate images.Photograph credit: Ivar Leidus