Peacock ore (iridescent chalcopyrite, possibly bornite)

A dark, metallic sulfide specimen with vivid purple, blue, green, yellow, and copper-colored iridescence. Chalcopyrite has Mohs hardness 3.5–4, metallic luster, a brassy-yellow fresh surface, poor-to-indistinct cleavage, and specific gravity about 4.1–4.3. Bornite is slightly softer, about Mohs 3, and commonly develops stronger purple-blue iridescent tarnish. The specimen appears to contain a dark massive matrix with numerous reflective sulfide grains; the photograph alone cannot conclusively distinguish chalcopyrite from bornite.

Forms mainly in hydrothermal copper deposits, magmatic sulfide systems, and porphyry-copper environments when copper-, iron-, and sulfur-rich fluids crystallize in fractures and veins. These minerals occur in rocks ranging from Precambrian to younger geologic ages, depending on the deposit. The multicolored coating develops through oxidation and tarnishing; strongly uniform rainbow colors on commercial specimens are often enhanced artificially.

Primarily an ore of copper and a source of associated sulfur and iron. Attractive pieces are sold as mineral specimens and occasionally used in inexpensive ornamental objects. It is generally too soft and brittle for durable jewelry, although polished pieces may be used as display stones.

“Peacock ore” is a commercial name rather than a single mineral species. Despite common claims that peacock ore is always bornite, many colorful specimens sold under that name are chalcopyrite, often acid-treated to intensify the iridescence. The rainbow effect is a thin surface tarnish layer that produces interference colors, not the true body color of the mineral.

In the field, look for a heavy, opaque, metallic specimen with brassy-yellow, bronze, or dark gray sulfide surfaces and rainbow tarnish. A fresh scratch or broken area should generally be brassy yellow rather than uniformly rainbow-colored. It may leave a dark greenish-black streak, is noticeably softer than quartz, and can be distinguished from pyrite by its lower hardness and more easily tarnished surface. Confirmation requires tests such as streak, hardness, specific gravity, reflected-light microscopy, or X-ray diffraction. Common localities include copper districts in Mexico, Peru, Chile, the United States, Canada, Cornwall, Kazakhstan, and China.