Chalcocite
Chalcocite, copper(I) sulfide (Cu2S), is an important copper ore mineral. It is opaque and dark-gray to black with a metallic luster. It has a hardness of 2½ - 3 on the Mohs scale. It is a sulfide with an orthorhombic crystal system.
The term chalcocite comes from the alteration of the obsolete name chalcosine, from the Greek khalkos, meaning copper. It is also known as redruthite, vitreous copper and copper-glance.