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A very hard, naturally occurring mineral composed of Aluminum oxide. Corundum is mined in Australia, South Africa, Zimbabwe, India, Russia, Rhodesia, Turkey, the United States and Canada (Ontario). The transparent to translucent stones are typically gray or brown with some deeply colored varieties. Transparent corundum stones have been used as gemstones since at least Hellenistic times. Rubies are transparent red corundum gems while transparent blue stones are called sapphires. Transparent yellow corundum is called Oriental topaz, the violet color is called Oriental amethyst and the green, Oriental emerald. Impure corundum, commonly called Emery, also contains Hematite, Magnetite, Silica and/or Magnesia. Emery is used as an abrasive. Artificial corundum has been produced on a very small scale by the Verneuil process.

(Corundum should not be confused with Carborundum)


Synonyms and Related Terms

emery (black); adamantine spar; ruby (red); sapphire (blue); Oriental topaz (yellow); Oriental amethyst (lavender); Oriental emerald (green); korund (Dan., Ned., Pol.); Korund (Deut.); corindon (Esp., Fr., Port.);




Inhalation of dust may cause irritation. Fire retardant.

Physical and Chemical Properties

Hexagonal crystal system with tabular, prismatic or pyramidal crystals.

Luster = vitreous to adamantine. Fracture = conchoidal. Streak = white

Fluorescence = orange to strong red. Heat treated stones may fluoresce green.

Natural stones contain microscopic mineral and fluid inclusions not seen in synthetic stones. Synthetic stones may have gas bubbles.

Composition Al2O3
CAS 1302-74-5
Mohs Hardness 8.9-9.0
Density 3.96-4.05
Refractive Index 1.761-1.769


Properties of Common Abrasives

Properties of Common Gemstones

Properties of Natural and Simulated Diamonds

Additional Images

Resources and Citations

  • Jack Odgen, Jewellery of the Ancient World, Rizzoli International Publications Inc., New York City, 1982
  • R.F.Symmes, T.T.Harding, Paul Taylor, Rocks, Fossils and Gems, DK Publishing, Inc., New York City, 1997
  • C.W.Chesterman, K.E.Lowe, Audubon Society Field Guide to North American Rocks and Minerals, Alfred A. Knopf, New York, 1979
  • CRC Handbook of Chemistry and Physics, Robert Weast (ed.), CRC Press, Boca Raton, Florida, v. 61, 1980 Comment: density=3.9-4.0
  • R. Mayer, The Artist's Handbook of Materials and Techniques, Viking Press, New York, 1981
  • G.S.Brady, Materials Handbook, McGraw-Hill Book Co., New York, 1971 Comment: p. 244
  • Richard S. Lewis, Hawley's Condensed Chemical Dictionary, Van Nostrand Reinhold, New York, 10th ed., 1993
  • Michael McCann, Artist Beware, Watson-Guptill Publications, New York City, 1979
  • R.M.Organ, Design for Scientific Conservation of Antiquities, Smithsonian Institution, Washington DC, 1968
  • Van Nostrand's Scientific Encyclopedia, Douglas M. Considine (ed.), Van Nostrand Reinhold, New York, 1976
  • Random House, Webster's Encyclopedic Unabridged Dictionary of the English Language, Grammercy Book, New York, 1997
  • The American Heritage Dictionary or Encarta, via Microsoft Bookshelf 98, Microsoft Corp., 1998

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