sulfides
Bornite
Cu5FeS4
- Also known as
- peacock orepurple copper ore
- Crystal system
- orthorhombic
- Hardness
- 3–3.25 on the Mohs scale
- Lustre
- metallic
- Colour
- Bronze when fresh, tarnishing to blue and purple
- Where it forms
- In copper deposits, usually with chalcopyrite and chalcocite.
- Chiefly from
- Butte, Montana, United StatesKazakhstanCornwall, EnglandChile
Why it is that colour
Fresh bornite is a dull copper-bronze. The blue-purple iridescence develops naturally over days to weeks as a thin tarnish film forms, and it is genuinely spontaneous, unlike the acid-treated chalcopyrite sold alongside it.
Safety
A copper sulfide. Safe to handle; do not powder or put in water.
Where it comes from
An important copper ore, and a specimen mineral. Genuine bornite is less common in shops than the chalcopyrite sold under its trade name.
Historical use
A significant copper ore in several major districts, notably Butte in Montana, which was one of the richest mining districts in the world and is now the site of the Berkeley Pit, a flooded open pit holding acidic metal-laden water.
Recorded as history and attributed where possible. That a stone was used for something establishes that people used it, not that it worked.
What is attributed to it, and since when
Named in 1845 after the Austrian mineralogist Ignaz von Born. No traditional attribution.
Described, not endorsed. Dating a claim is not the same as assessing it — but an association that first appears in a book published in 1985 is not an ancient inheritance, whatever the label says. A sincere account of feeling better is compatible with the stone doing nothing — see the placebo effect.
Worth knowing
Bornite is the real peacock ore, and its colours arrive on their own. A freshly broken surface is plain bronze; leave it exposed and over a few weeks a tarnish film builds up and the blues and purples appear. That is a genuine and rather pleasant thing for a mineral to do.
It is also why the substitution happened. Natural bornite iridescence is subtler than the electric blue-and-purple of acid-treated chalcopyrite, so the fake looks more like the reputation than the real thing does.
Identity checked against IMA-CNMNC master list, June 2025.