Bornite is a copper iron sulfide mineral (Cu5FeS4). It's one of several economically significant copper ore minerals (others include chalcocite and chalcopyrite). On fresh, unweathered surfaces, bornite has a metallic copper-orange appearance. Fresh surfaces tarnish relatively quickly. Early-formed bornite tarnish is iridescent, with blues and purples and reds and greens, resulting in the nickname “peacock ore”. As the tarnish thickens, more blues and purples stand out. Late-stage bornite tarnish is a dark purplish-blue. The tarnish material is actually covellite (CuS). With weathering, oxidation, and breakdown, bornite converts to covellite and chalcocite.
Bornite is moderately soft (H=3), has no cleavage, and is noticeably heavy for its size.
Bornite (field of view ~1.8 cm across). Photo by Nicole Byrd.
Bornite (all but the yellow brassy areas - that's chalcopyrite) (field of view ~3.1 cm across) from Butte, southwestern Montana, USA. The bronzy-gray areas are purplish on the actual specimen (photographing faithful colors on metallic minerals can be difficult).