Bornite is an important copper ore mineral. It ranks up there with many of the other copper ores such as chalcocite, chalcopyrite, covellite, digenite, cuprite and tetrahedrite. As a mineral specimen, bornite is usually lacking. Good crystals are rare and thus bornite is commonly known as simply a massive mineral ore.
Also called "peacock ore". Peacock ore or alternatively peacock copper has a rich bouquet of colours. The colours are from an iridescent tarnish that forms on bornite upon exposure to air. The tarnish is made of assorted copper oxides or hydroxides that form a mere atoms thin layer over the bornite.
This effect is the same as the rainbow effect that occurs with oil on water. In the case of bornite, the tarnish will have a purplish, violet or blue colour. Because bornite is often intergrown with chalcopyrite which tarnishes to more greens and yellows, the peacock ore may have many colours ranging from purple to blue to green to yellow.
Bornite's crystals, if found, are usually distorted cubes with curved faces. Even rarer are the distorted octahedrons and dodecahedrons. These are isometric crystals. However, bornite's structure at normal temperatures is not isometric. Bornite is only isometric at temperatures above 228 degrees celsius and it was above this temperature that the crystals formed. As bornite cooled it structurally altered to possibly a tetragonal structure but outwardly it retained the isometric forms.