oxides
Chromite
Fe2+Cr2O4
- Crystal system
- cubic
- Hardness
- 5.5 on the Mohs scale
- Lustre
- metallic
- Colour
- Black to brownish black
- Where it forms
- In layered ultramafic intrusions, where it crystallised early from cooling magma and settled into distinct layers.
- Chiefly from
- Bushveld Complex, South AfricaKazakhstanTurkeyIndia
Why it is that colour
Iron and chromium oxide. Opaque and dark, with a brown streak.
Safety
Chromite itself contains chromium in the trivalent state, which is an essential trace nutrient and comparatively benign. Hexavalent chromium, produced in some industrial processing, is a serious carcinogen. Handling specimens is safe; the hazard belongs to processing plants and to contaminated groundwater near them.
Where it comes from
South Africa and Kazakhstan dominate. Chromium is essential to stainless steel, which makes chromite a strategic mineral, and the Bushveld Complex holds most of the world reserve.
Historical use
Chromium takes its name from the Greek for colour, because its compounds are vividly coloured — and chromium is the impurity that makes ruby red and emerald green. The element that colours the two most valuable gemstones comes out of a black, dull, unremarkable ore.
Recorded as history and attributed where possible. That a stone was used for something establishes that people used it, not that it worked.
Worth knowing
Chromium is the connecting thread through a surprising number of entries in this collection. It makes ruby red in corundum, emerald green in beryl, tsavorite green in grossular, uvarovite green in garnet, and it does all of that from an element mined as this dull black oxide.
Whether chromium produces red or green depends on the crystal field around it — the geometry and spacing of the atoms it sits among. Same element, same trace quantity, opposite ends of the spectrum, and the difference is the house it lives in rather than the guest.
Identity checked against IMA-CNMNC master list, June 2025.