Color is normally black, but also brownish black to dark gray. Luster is submetallic or resinous. Transparency crystals are opaque. Also as platy crystals and in massive, compact and columnar specimens. Cleavage very poor, in one direction lengthwise.
Fracture is conchoidal.
Associated Minerals are arsenopyrite , stilbite , hematite , magnetite and pyrite . Best Field Indicators crystal habit, color, hardness, cleavage and localities.