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Several corundum crystals.jpg
CategoryOxide mineral – Hematite group
(repeating unit)
Aluminium oxide, Al
Strunz classification4.CB.05
Dana classification4.3.1.1
Crystal systemTrigonal
Crystal classHexagonal scalenohedral (3m)
H-M symbol: (3 2/m)
Space groupR3c
Unit cella = 4.75 Å, c = 12.982 Å; Z = 6
ColorColorless, gray,, golden-brown, brown; purple, pink to red, orange, yellow, green, blue, violet; may be color zoned, asteriated mainly grey and brown
Crystal habitSteep bipyramidal, tabular, prismatic, rhombohedral crystals, massive or granular
TwinningPolysynthetic twinning common
CleavageNone – parting in 3 directions
FractureConchoidal to uneven
Mohs scale hardness9 (defining mineral)[1]
LusterAdamantine to vitreous
DiaphaneityTransparent, translucent to opaque
Specific gravity3.95–4.10
Optical propertiesUniaxial (–)
Refractive indexnω = 1.767–1.772
nε = 1.759–1.763
Melting point2,044 °C (3,711 °F)
Alters toMay alter to mica on surfaces causing a decrease in hardness
Other characteristicsMay fluoresce or phosphoresce under UV light
Major varieties
SapphireAny color except red
EmeryBlack granular corundum intimately mixed with magnetite, hematite, or hercynite

Corundum is a crystalline form of aluminium oxide (Al
) typically containing traces of iron, titanium, vanadium and chromium.[2][3] It is a rock-forming mineral. It is also a naturally transparent material, but can have different colors depending on the presence of transition metal impurities in its crystalline structure.[6] Corundum has two primary gem varieties: ruby and sapphire. Rubies are red due to the presence of chromium, and sapphires exhibit a range of colors depending on what transition metal is present.[6] A rare type of sapphire, padparadscha sapphire, is pink-orange.

The name "corundum" is derived from the Tamil-Dravidian word kurundam (ruby-sapphire) (appearing in Sanskrit as kuruvinda).[7]

Because of corundum's hardness (pure corundum is defined to have 9.0 on the Mohs scale), it can scratch almost every other mineral. It is commonly used as an abrasive on everything from sandpaper to large tools used in machining metals, plastics, and wood. Some emery is a mix of corundum and other substances, and the mix is less abrasive, with an average Mohs hardness of 8.0.

In addition to its hardness, corundum has a density of 4.02 g/cm3 (251 lb/cu ft), which is unusually high for a transparent mineral composed of the low-atomic mass elements aluminium and oxygen.[8]

Geology and occurrence[edit]

Corundum from Brazil, size about 2 cm × 3 cm (0.8 in × 1 in).

Corundum occurs as a mineral in mica schist, gneiss, and some marbles in metamorphic terranes. It also occurs in low silica igneous syenite and nepheline syenite intrusives. Other occurrences are as masses adjacent to ultramafic intrusives, associated with lamprophyre dikes and as large crystals in pegmatites.[5] It commonly occurs as a detrital mineral in stream and beach sands because of its hardness and resistance to weathering.[5] The largest documented single crystal of corundum measured about 65 cm × 40 cm × 40 cm (26 in × 16 in × 16 in), and weighed 152 kg (335 lb).[9] The record has since been surpassed by certain synthetic boules.[10]

Corundum for abrasives is mined in Zimbabwe, Pakistan, Afghanistan, Russia, Sri Lanka, and India. Historically it was mined from deposits associated with dunites in North Carolina, US and from a nepheline syenite in Craigmont, Ontario.[5] Emery-grade corundum is found on the Greek island of Naxos and near Peekskill, New York, US. Abrasive corundum is synthetically manufactured from bauxite.[5] Four corundum axes dating back to 2500 BCE from the Liangzhou culture have been discovered in China.[11]

Synthetic corundum[edit]

Antoine Lavoisier was the first to synthesize corundum in 1782, by using an oxygen blowpipe to heat a sample of alumina upon a bed of burning charcoal.[12] However, it was not until 1820 that Edward Daniel Clarke identified the result of the experiment as corundum.[13] Marc Gaudin would later heat ammonium alum and potassium chromate under an oxyhydrogen blowpipe to produce the first synthetic ruby in 1834.[14]

In 1847 Jacques-Joseph Ébelmen took a different approach to the crystallisation of corundum: crystallising the material from a solution of alumina in boric acid, rather than from a melt.[15] Furthermore, by adding chromium oxide to his solution, Ébelmen became the first person to synthesize transparent ruby.[15] However, although Ébelmen's rubies were transparent, they were also microscopic.[15]

Edmond Frémy would later improve the crystallisation of ruby from a solution to grow larger crystals: first alongside the industrial glass-maker Charles Feil, and latterly alongside his student Auguste Verneuil.[16] Similarly, Verneuil would later improve the crystallisation of ruby from a melt. After Verneuil published details of his more efficient method of flame fusion (now known as the Verneuil process) in 1902, the industrial production of synthetic corundum could begin.

The Verneuil process allows the production of flawless single-crystal sapphire and ruby gems of much larger size than normally found in nature. It is also possible to grow gem-quality synthetic corundum by flux-growth and hydrothermal synthesis. Because of the simplicity of the methods involved in corundum synthesis, large quantities of these crystals have become available on the market causing a significant reduction of price in recent years. Apart from ornamental uses, synthetic corundum is also used to produce mechanical parts (tubes, rods, bearings, and other machined parts), scratch-resistant optics, scratch-resistant watch crystals, instrument windows for satellites and spacecraft (because of its transparency in the ultraviolet to infrared range), and laser components.

Structure and physical properties[edit]

Crystal structure of corundum
Molar volume vs. pressure at room temperature.

Corundum crystallizes with trigonal symmetry in the space group R3c and has the lattice parameters a = 4.75 Å and c = 12.982 Å at standard conditions. The unit cell contains six formula units.

The toughness of corundum is sensitive to surface roughness[17][18] and crystallographic orientation.[19] It may be 6–7 MPa·m12 for synthetic crystals,[19] and around 4 MPa·m12 for natural.[20]

In the lattice of corundum, the oxygen atoms form a slightly distorted hexagonal close packing, in which two-thirds of the gaps between the octahedra are occupied by aluminium ions.


  1. ^ "Mohs' scale of hardness". Collector's corner. Mineralogical Society of America. Retrieved 10 January 2014.
  2. ^ a b Anthony, John W.; Bideaux, Richard A.; Bladh, Kenneth W.; Nichols, Monte C., eds. (1997). "Corundum". Handbook of Mineralogy (PDF). III(Halides, Hydroxides, Oxides). Chantilly, VA, US: Mineralogical Society of America. ISBN 0962209724.
  3. ^ a b Corundum.
  4. ^ Corundum Archived 2006-11-25 at the Wayback Machine. Webmineral
  5. ^ a b c d e Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., Wiley, pp. 300–302 ISBN 0-471-80580-7
  6. ^ a b Giuliani, Gaston; Ohnenstetter, Daniel; Fallick, Anthony E.; Groat, Lee; Fagan; Andrew J (2014). "The Geology and Genesis of Gem Corundum Deposits". Gem Corundum. Research Gate: Mineralogical Association of Canada. pp. 37–38. ISBN 978-0-921294-54-2.
  7. ^ Harper, Douglas. "corundum". Online Etymology Dictionary.
  8. ^ The Mineral Corundum.
  9. ^ Rickwood, P. C. (1981). "The largest crystals" (PDF). American Mineralogist. 66: 885–907.
  10. ^ Rubicon Technology Grows 200kg "Super Boule", LED Inside, April 21, 2009
  11. ^ "Chinese made first use of diamond". BBC. BBC. May 2005.
  12. ^ Evans, James (2020). The History of Synthetic Ruby. Lustre Gemmology. pp. 3–8. ISBN 1916165206.
  13. ^ Evans, James (2020). The History of Synthetic Ruby. Lustre Gemmology. pp. 9–13. ISBN 1916165206.
  14. ^ Evans, James (2020). The History of Synthetic Ruby. Lustre Gemmology. pp. 13–15. ISBN 1916165206.
  15. ^ a b c Evans, James (2020). The History of Synthetic Ruby. Lustre Gemmology. p. 16. ISBN 1916165206.
  16. ^ Evans, James (2020). The History of Synthetic Ruby. Lustre Gemmology. pp. 17–27. ISBN 1916165206.
  17. ^ Farzin-Nia, Farrokh; Sterrett, Terry; Sirney, Ron. "Effect of machining on fracture toughness of corundum". Journal of Materials Science. 25 (5): 2527–2531. doi:10.1007/bf00638054.
  18. ^ "Fracture-Strength Anisotropy of Sapphire". Journal of the American Ceramic Society. 59: 59–61. doi:10.1111/j.1151-2916.1976.tb09390.x.
  19. ^ a b "Fracture of Sapphire". Journal of the American Ceramic Society. 52: 485–491. doi:10.1111/j.1151-2916.1969.tb09199.x.
  20. ^ "Corundum, Aluminum Oxide, Alumina, 99.9%, Al 2 O 3".