Mineral · Hematite group
Corundum
Al2O3
Corundum is aluminium oxide, hardness 9, whose red variety is ruby and every other colour sapphire; specimen crystals are barrel-shaped hexagonal prisms from Mogok (Myanmar), Sri Lanka, Madagascar, Winza (Tanzania), Montana (Yogo, Rock Creek) and India (ruby in zoisite). Opaque ruby crystals in marble or fuchsite are cheap; transparent crystals are priced as gems. Its hardness scratches everything but diamond.
| Formula | Al2O3 |
|---|---|
| IMA status | grandfathered |
| Crystal system | Trigonal |
| Mohs hardness | 9 |
| Group | Hematite group |
| Type locality | None (known since antiquity) |
| Streak | White |
| Specific gravity | 4.0 |
| Reference list | IMA List of Minerals (RRUFF) — verify formula and status against the current list |
Fakes and treatments
Synthetic corundum (Verneuil boules) has been sold as natural since 1900; heat, glass filling and diffusion treatments are routine in cut stones. Indian 'ruby' crystals are often dyed or oiled to deepen colour.
Elsewhere
Cite this article
Stratum Study. "Corundum" (Mineral). Stratum Study pack 2026.09, as of 2026-09-19. https://stratum.black/study/minerals/corundum. Accessed 2026-09-20.
@misc{stratum-corundum,
title = {Corundum},
author = {{Stratum Study}},
howpublished = {\url{https://stratum.black/study/minerals/corundum}},
year = {2026},
note = {Mineral article, Stratum Study pack 2026.09, as of 2026-09-19; sources: stratum, wikidata, commons. Accessed 2026-09-20.}
} Citations carry the pack version and the article's as-of date so they resolve to the same text later. The facts themselves belong to the sources listed below; cite those for the science.