What does corundum mean?
Corundum is a naturally occurring mineral composed of aluminium oxide, crystallising into one of the hardest known substances — only diamond tops it among common natural minerals on the Mohs scale, where corundum rates nine. In its pure state it is colourless, but trace impurities produce some of the world's most prized gemstones: chromium yields the red ruby, while iron and titanium produce the variously coloured sapphires. Beyond jewellery, corundum's exceptional hardness and resistance to heat make it industrially valuable as an abrasive in grinding wheels, sandpaper, and polishing compounds, and as a refractory material in furnaces. The terms 'emery' and carborundum are sometimes met with in similar contexts, though they denote related but distinct abrasives. For readers, corundum is a useful word for understanding both the science of minerals and the language of the gem trade.
nounAn extremely hard crystalline form of aluminium oxide (Al₂O₃) that occurs naturally as an igneous and metamorphic mineral; transparent varieties include ruby and sapphire. It ranks 9 on the Mohs scale of hardness and is widely used as an abrasive.
- The mineral aluminium oxide (Al₂O₃) in its natural crystalline form, valued for extreme hardness and used as an abrasive and refractory material.
- The mineral species to which the gemstones ruby and sapphire belong.
"The geologist identified the rough red crystals as corundum, the mineral that yields rubies when gem-quality."
"Emery paper owes its cutting power to grains of corundum bonded to its surface."
"Chemically speaking, a sapphire is simply blue corundum, tinted by traces of iron and titanium."
As a mass noun, 'corundum' normally has no plural. The plural 'corundums' appears chiefly in geological or mineralogical contexts referring to individual crystals or specimens.
"The thin sections revealed scattered corundums embedded in the marble."
Rubies and sapphires are the same mineral — corundum — differing only in trace impurities: chromium makes a ruby, iron and titanium make a sapphire.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of corundum
The word corundum entered English in the early 18th century, borrowed from Tamil kuruvindam (or a closely related Indian source), reflecting the fact that many of the finest rubies and sapphires historically reached Europe from South Asia and Ceylon. It thus belongs to the small family of English mineral names drawn directly from Indian languages. Its adoption coincided with the growing scientific study of gems and minerals in Europe.
Related word forms
How corundum is actually used
Corundum is a technical term used mainly in mineralogy, geology, and industry rather than everyday conversation. It is treated as an uncountable mass noun when referring to the mineral in general, though 'corundums' can appear when referring to distinct crystals or specimens. In gemmological contexts it is often discussed by variety name (ruby, sapphire) rather than as corundum itself.
Easily confused with corundum
Carborundum is a trade name for silicon carbide, a man-made abrasive, whereas corundum is a naturally occurring mineral of aluminium oxide.
A conundrum is a difficult problem or riddle, while corundum is a hard mineral — the words merely sound similar.