Bornite
Bornite is a copper iron sulfide mineral with the formula Cu5FeS4, prized by mineral collectors for the flashy iridescent tarnish that earns it the nickname peacock ore. Freshly broken surfaces show a coppery, brownish-bronze metallic color that oxidizes within hours to purple, blue, and pink iridescence. It is an important, though secondary, copper ore mineral found in many sulfide deposits worldwide.
Bornite properties at a glance
| Type | Minerals |
|---|---|
| Chemical formula | Cu5FeS4 |
| Mohs hardness | 3–3.25 |
| Crystal system | Orthorhombic (pseudocubic) |
| Luster | Metallic |
| Streak | Grayish black |
| Cleavage | Poor, indistinct octahedral |
| Fracture | Conchoidal to uneven |
| Specific gravity | 4.9–5.3 |
| Transparency | Opaque |
| Common colors | Copper red to brownish bronze (fresh), Iridescent purple-blue tarnish (peacock ore) |
| Rarity | common |
Geology & background
Bornite forms in hydrothermal veins, contact metamorphic zones, and as a primary mineral in porphyry copper deposits, commonly intergrown with chalcopyrite, chalcocite, and pyrite. Notable localities producing fine crystallized specimens include Butte, Montana; the Kennecott deposits of Alaska; and Chessy, France, along with significant occurrences in Peru, Chile, and Cornwall, England, where bornite has been mined alongside other copper sulfides for centuries.
Collectors specifically seek out freshly broken or acid-etched pieces because the vivid tarnish colors fade with prolonged air exposure, so many dealers store specimens away from light and touch to preserve brilliance. Bornite is often intergrown with chalcopyrite, and distinguishing the two on a tarnished piece can be difficult without a fresh fracture surface. As a secondary but economically significant copper ore, bornite also carries real industrial value beyond the display case.
How to identify Bornite
Bornite shows a metallic luster and a distinctive coppery-bronze color on fresh breaks that rapidly tarnishes to iridescent purple, blue, and pink hues, the trait most collectors use for quick field identification. It has a hardness of about 3, a grayish-black streak, and no visible cleavage, breaking instead with a conchoidal to uneven fracture. Bornite is commonly confused with chalcopyrite and with tarnished chalcocite, but its softer hardness and more colorful iridescence usually distinguish it once a streak test is performed.
- Compare the specimen's color, luster, crystal habit, grain size, streak, and locality before confirming.
- Use multiple photos when possible, including a fresh surface and a close view of texture.
Where Bornite is found
Reported from multiple localities worldwide; verify locality against specimen provenance when available.
How much is Bornite worth?
| Entry-level specimen | $2 |
|---|---|
| Typical collector specimen | $20 |
| Premium specimen | $150+ |
Value depends on size, crystal quality, color, locality, rarity, damage, and provenance.
Common uses
- Collector specimens
- Education
- Field identification
- Industrial or lapidary use when suitable
Interesting facts
- Bornite's rainbow tarnish, which forms within hours of exposure to air, gives it the popular nickname peacock ore.
- Despite the flashy tarnish, bornite's true fresh-surface color is a plain coppery bronze rather than any iridescent hue.
- Bornite can contain over 60 percent copper by weight, making it richer in copper than the more common chalcopyrite.
Care & safety
Handle as a collector specimen until hardness, cleavage, and fracture are verified.
Avoid inhaling dust. Do not cut, grind, or polish unknown specimens without eye protection and a proper respirator.
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