A yellow sulphide of copper and iron, crystallizing in the tetragonal system; the principal ore of copper. It is similar in appearance to pyrite and gold.

It crystallizes in igneous rocks and hydrothermal veins associated with the upper parts of acid igneous intrusions It is the most widespread of the copper ores, occurring e.g. in Cornwall (UK), Sudbury (Canada), Chile, Tasmania (Australia), and Rio Tinto (Spain).