English: Chalcocite, Cu2S (copper sulfide), is often found with chalcopyrite CuFeS2. Chalcocite is shiny and lead-gray when freshly broken, but turns black and becomes sooty when exposed to air. Chalcopyrite is yellow, sometimes slightly orangish or with a hint of green, when fresh. It may tarnish, as seen in this sample. Chalcocite and chalcopyrite are both important sources of copper (and iron).