The two minerals are in a series in which the sulfur and selenium ions substitute for each other. Clausthalite and the mineral crookesite , a copper thallium selenide are the two most common selenium minerals. However they are not ores of selenium due to their rarity and the fact that selenium is mostly acquired through the processing of copper sulfide ores. The selenium is found as a trace in many copper sulfide minerals especially pyrite and in coal. The element selenium is used in the production of photoelectric cells, resisters, photographic chemicals, pigments and many other industrial uses.
Color is a bright lead gray, blue gray to gray black.
Luster is metallic. Crystals are opaque. Hardness is 2.5. Streak is black. Associated Minerals include gold , pyrite , calcite , epidote , selenium , sphalerite , tiemannite , naumannite , penroseite , galena and other sulfides .