Corundum
Species
Uncommon · Al₂O₃
Aluminium oxide, the hardest common mineral after diamond, at 9 on the Mohs scale. Pure corundum is colourless; chromium produces ruby, while iron and titanium together produce blue sapphire. Used industrially as an abrasive.
Identity
- Formula
- Al₂O₃
- Chemical class
- Oxide
- Varieties
- Ruby, Sapphire
- Also known as
- Emery (corundum-rich rock), Ruby (red gem variety), Sapphire (other gem colors)
Crystallography
- Crystal system
- Trigonal
- Habits
- Tabular, Prismatic, Rhombohedral, Pyramidal
- Fracture
- Conchoidal
Physical properties
- Mohs hardness
- 9
- Density
- 4.02 g/cm³
- Luster
- Vitreous, Adamantine
- Colors
- gray, brown, red, blue, yellow, colorless
- Streak
- white
- Transparency
- Transparent, Translucent, Opaque
Geology
- Origins
- Metamorphic, Igneous
- Host rocks
- Granite, Schist and gneiss, Marble
- Associated minerals
- Spinel, Rutile, Mica, Calcite, Kyanite, Chlorite
- Geography
- worldwide
Classification
- Rarity
- Uncommon
- Gem quality
- Yes
Sources
- M. Pasero and others, IMA Commission on New Minerals, Nomenclature and Classification, The New IMA List of Minerals, updated September 2026. International Mineralogical Association (CNMNC), 2026.
- Corundum. Mindat.org (Hudson Institute of Mineralogy).
- J. W. Anthony, R. A. Bideaux, K. W. Bladh, M. C. Nichols, Corundum. Mineral Data Publishing, Handbook of Mineralogy, 2001.
- Ruby. Mindat.org (Hudson Institute of Mineralogy).
- Sapphire. Mindat.org (Hudson Institute of Mineralogy).
- Ruby Description. Gemological Institute of America, Gem Encyclopedia.
- Sapphire Description. Gemological Institute of America, Gem Encyclopedia.
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