oxides
Corundum
Al2O3
- Also known as
- rubysapphireemery
- Crystal system
- trigonal
- Hardness
- 9 on the Mohs scale
- Lustre
- adamantine
- Colour
- Colourless when pure; red as ruby, any other colour as sapphire
- Where it forms
- In aluminium-rich, silica-poor metamorphic and igneous rocks. It cannot form where there is enough silica to make a feldspar instead, which is why gem corundum is uncommon.
- Chiefly from
- MyanmarSri LankaMadagascarMontana, United StatesAustralia
Why it is that colour
Pure aluminium oxide is colourless. Chromium gives red, iron and titanium together give blue, and other combinations give every other colour the trade sells.
Safety
No hazard from handling. Fine corundum dust is an abrasive nuisance rather than a toxin.
Where it comes from
Gem corundum comes overwhelmingly from Myanmar, Sri Lanka, Madagascar, Mozambique and Thailand. Burmese ruby has been subject to international sanctions at various points because of the military regime’s control of the mines. Industrial corundum is now almost entirely synthetic.
Historical use
Second only to diamond in hardness on the Mohs scale, and for most of history the hardest abrasive available. Emery — impure corundum with magnetite — was the standard polishing abrasive of the ancient world and gives its name to emery boards and emery cloth.
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
Ruby and sapphire are both ancient and both extensively documented in Sanskrit, Greek, Roman and biblical sources. Corundum as a mineral name is modern, from Sanskrit kuruvinda by way of Tamil.
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
Corundum is the reason the Mohs scale has an awkward gap at the top. Mohs numbered ten minerals in order of which scratches which, and the intervals are not equal: corundum at 9 is roughly twice as hard as topaz at 8, and diamond at 10 is around four times harder than corundum. The scale tells you the order, not the distance.
Synthetic corundum has been made since Auguste Verneuil perfected his flame-fusion process in 1902, and it is one of the great unremarked industrial successes. Every mechanical watch runs on synthetic ruby bearings. The stones in the movement of a watch described as "17 jewels" are laboratory corundum, and they are there because it is hard, not because it is precious.
Identity checked against IMA-CNMNC master list, June 2025.