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Several corundum crystals.jpg
Category Oxide mineral – Hematite group
Chemical formula Aluminium oxide, Al2O3
Color Colorless, gray, brown; pink to red, orange, yellow, green, blue, violet; may be color zoned, asteriated mainly grey and brown
Crystal habit Steep bipyramidal, tabular, prismatic, rhombohedral crystals, massive or granular
Crystal system Trigonal (Hexagonal Scalenohedral)
Symbol (32/m)
Space group: R3c
Twinning Polysynthetic twinning common
Cleavage None – parting in 3 directions
Fracture Conchoidal to uneven
Tenacity Brittle
Mohs Scale hardness 9 (defining mineral)[1]
Luster Adamantine to vitreous
Refractive index nω = 1.767–1.772
nε = 1.759–1.763
Optical Properties Uniaxial (–)
Streak White
Specific gravity 3.95–4.10
Melting point 2044 °C
Fusibility Infusible
Solubility Insoluble
Diaphaneity Transparent, translucent to opaque
Other Characteristics May fluoresce or phosphoresce under UV light
Major varieties
Sapphire Any color except red
Ruby Red
Emery Black granular corundum intimately mixed with magnetite, hematite, or hercynite

Corundum is a mineral. It is very hard, with a Mohs hardness of 9 (diamond is 10). It is clear. Its chemical formula is aluminium oxide, Al2O3. Sometimes some of the aluminium atoms get replaced with chromium atoms. It then turns into ruby.

Related pages[change | change source]

References[change | change source]

  1. "Mohs' scale of hardness". Collector's corner. Mineralogical Society of America. Retrieved 10 January 2014.
  2. Anthony, John W.; Bideaux, Richard A.; Bladh, Kenneth W. and Nichols, Monte C., ed. (1997). "Corundum" (PDF). Handbook of Mineralogy. III(Halides, Hydroxides, Oxides). Chantilly, VA, US: Mineralogical Society of America. ISBN 0962209724.
  3. Corundum.
  4. Corundum. Webmineral
  5. Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., Wiley, pp. 300–302 ISBN 0-471-80580-7