Covellite (also know as covelline) is a crystalline form of copper sulfide (CuS). In appearance, covellite is an enchanting indigo blue.
As described for the mineral pyrite, the assignment of formal oxidation states (or charges) to the atoms that constitute covellite is non-obvious. The formula might seem to suggest the description Cu2+, S2-. In fact the atomic structure shows that copper and sulfur each adopt two different geometries. Thus, the mineral is probably best described as consisting of both Cu2+, S2- and 2Cu+, S22-.
The mineral is associated with chalcosite in zones of secondary enrichment (supergene) of copper sulfide deposits. Commonly found with and as coatings on chalcocite, chalcopyrite, bornite, enargite, pyrite, and other sulfides. It often occurs as pseudomorphic replacements after other minerals. Very rare occurrence as a volcanic sublimate at Mount Vesuvius the site of its discovery or first description by Niccola Covelli (1790 - 1829).