Covelline, also called covellite, is a copper sulphide (CuS) mineral with a characteristic iridescent indigo-blue colour. It rarely occurs in its pure state and is more important as a collector??? mineral than as an ore of copper. It is most commonly found as a coating on, or mixed with, other copper sulphates such as chalcocite, bornite, pyrite and others. Covelline is always a secondary mineral in copper rich deposits, either in the form of rosettes or platy crystals, but more often in massive, non crystalline form. This specimen is from the Butte Mining district, Montana, USA. Actual width 100 millimetres.