Category Oxide mineral – Hematite group
Chemical formula Aluminium oxide, Al2O3
Strunz classification 04.CB.05
Dana classification
Crystal symmetry Trigonal (32/m)
Unit cell a = 4.75 Å, c = 12.982 Å; Z=6
Color Colorless, gray, brown; pink to pigeon-blood-red, orange, yellow, green, blue to cornflower 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
Mohs scale hardness 9 (defining mineral)
Luster Adamantine to vitreous
Streak White
Diaphaneity Transparent, translucent to opaque
Specific gravity 3.95–4.10
Optical properties Uniaxial (–)
Refractive index nω = 1.767–1.772
nε = 1.759–1.763
Pleochroism None
Melting point 2044 °C
Fusibility Infusible
Solubility Insoluble
Alters to May alter to mica on surfaces causing a decrease in hardness
Other characteristics May fluoresce or phosphoresce under UV
References [1][2][3][4]
Major varieties
Sapphire Any color except red
Ruby Red
Emery Black granular corundum intimately mixed with magnetite, hematite, or hercynite

Corundum is a crystalline form of aluminium oxide (Al2O3) with traces of iron, titanium and chromium.[1] It is a rock-forming mineral. It is one of the naturally clear transparent materials, but can have different colors when impurities are present. Transparent specimens are used as gems, called ruby if red and padmaraga if pink-orange. All other colors are called sapphire, e.g., "green sapphire" for a green specimen.

The name "corundum" is derived from the Tamil word குருந்தம் "kuruntam" meaning "ruby", and related to Sanskrit "kuruvinda".[3]

Because of corundum's hardness (pure corundum is defined to have 9.0 Mohs), it can scratch almost every other mineral. It is commonly used as an abrasive, on everything from sandpaper to large machines 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 hardness near 8.0.

In addition to its hardness, corundum is unusual for its density of 4.02 g/cm3, which is very high for a transparent mineral composed of the low atomic mass elements aluminium and oxygen.[5]

Geology and occurrence

Corundum from Brazil, size about 2×3 cm.

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.[4] It commonly occurs as a detrital mineral in stream and beach sands because of its hardness and resistance to weathering.[4] The largest documented single crystal of corundum measured about 65×40×40 cm.[6]

Corundum for abrasives is mined in Zimbabwe, Russia, Sri Lanka and India. Historically it was mined from deposits associated with dunites in North Carolina, USA and from a nepheline syenite in Craigmont, Ontario.[4] Emery grade corundum is found on the Greek island of Naxos and near Peekskill, New York, USA. Abrasive corundum is synthetically manufactured from bauxite.[4]

Synthetic corundum

In 1837, Gaudin made the first synthetic rubies by fusing alumina at a high temperature with a small amount of chromium as a pigment. In 1847, Ebelmen made white sapphires by fusing alumina in boric acid. In 1877 Frenic and Freil made crystal corundum from which small stones could be cut. Frimy and Auguste Verneuil manufactured artificial ruby by fusing BaF2 and Al2O3 with a little chromium at temperatures above 2,000 °C (3,632 °F). In 1903, Verneuil announced he could produce synthetic rubies on a commercial scale using this flame fusion process.[7]

Crystal structure of corundum

The Verneuil process allows the production of flawless single-crystal sapphires, rubies and other corundum 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 from the UV to IR), and laser components.


  1. ^ a b Handbook of Mineralogy
  2. ^ Corundum at
  3. ^ a b Corundum at Webmineral
  4. ^ 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
  5. ^ Mineral Galleries
  6. ^ P. C. Rickwood (1981). "The largest crystals". American Mineralogist 66: 885–907. 
  7. ^ "Bahadur: a Handbook of Precious Stones". 1943. Retrieved 2007-08-19. 

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Look at other dictionaries:

  • Corundum — Co*run dum (k[ o]*r[u^]n d[u^]m), n.; pl. {Corundums} (k[ o]*r[u^]n d[u^]mz). [Also corindon.] [From Hind. kurand corundum stone.] (Min.) The mineral alumina ({Al2O3}), as found native in a crystalline state. Transparent varieties are used as… …   The Collaborative International Dictionary of English

  • corundum — very hard mineral, 1728, from Anglo Indian, from Tamil kurundam ruby sapphire (Skt. kuruvinda), of unknown origin …   Etymology dictionary

  • corundum — ► NOUN ▪ extremely hard crystallized alumina, used as an abrasive. ORIGIN Tamil …   English terms dictionary

  • corundum — [kə run′dəm] n. [Tamil kurundam < Sans kuruvinda, ruby] a very hard mineral, aluminum oxide, Al2O3: a dark, coarse variety ( emery) is used for grinding and polishing; clear, transparent varieties ( ruby, sapphire) are used as gems: see MOHS… …   English World dictionary

  • corundum — /keuh run deuhm/, n. a common mineral, aluminum oxide, Al2O3, notable for its hardness: transparent varieties, as sapphire and ruby, are used as gems, other varieties as abrasives: often made synthetically. [1720 30; < Tamil kuruntam; akin to Skt …   Universalium

  • corundum — korundas statusas T sritis radioelektronika atitikmenys: angl. corundum vok. Korund, m rus. корунд, m pranc. corindon, m …   Radioelektronikos terminų žodynas

  • corundum — korundas statusas T sritis chemija apibrėžtis Mineralas. formulė Al₂O₃ atitikmenys: angl. corundum rus. корунд …   Chemijos terminų aiškinamasis žodynas

  • corundum — noun Etymology: Tamil kuruntam; akin to Sanskrit kuruvinda ruby Date: 1804 a very hard mineral that consists of aluminum oxide occurring in massive and crystalline forms, that can be synthesized, and that is used for gemstones (as ruby and… …   New Collegiate Dictionary

  • corundum — noun an extremely hard mineral, a form of aluminium oxide with the chemical formula AlO, that occurs in the form of the gemstones ruby and sapphire; it is used as an abrasive …   Wiktionary

  • corundum — Native crystalline aluminum oxide. [Hind. kurand] * * * co·run·dum kə rən dəm n a very hard mineral Al2O3 that consists of alumina occurring in massive form and as variously colored crystals which include the ruby and sapphire * * * co·run·dum… …   Medical dictionary

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