Acanthite (a.k.a. argentite) is a scarce silver sulfide mineral - Ag2S. On fresh surfaces, acanthite has a metallic silvery-gray appearance. On less fresh surfaces, it's got a dull metallic luster and a dark gray to black-colored tarnish. Acanthite lacks cleavage, has a black streak, is fairly soft (H = 2 to 2.5), and is fairly heavy for its size. Acanthite is the most important silver ore mineral.
Acanthite (argentite) (9 mm across along the base) from the Imiter Mine, Imiter Silver Deposit, Anti-Atlas Mountains, central Morocco. Ore mineralization at Imiter has been dated to ~550 million years (near-latest Neoproterozoic).