Corundum is a crystalline form of aluminium oxide (Al
) typically containing traces of iron, titanium, vanadium and chromium.[3][4] It is a rock-forming mineral. It is a naturally transparent material, but can have different colors depending on the presence of transition metal impurities in its crystalline structure.[7] 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.[7] A rare type of sapphire, padparadscha sapphire, is pink-orange.

CategoryOxide mineral – Hematite group
(repeating unit)
Aluminium oxide, Al
IMA symbolCrn[1]
Strunz classification4.CB.05
Dana classification4.3.1.1
Crystal systemTrigonal
Crystal classHexagonal scalenohedral (3m)
H-M symbol: (3 2/m)
Space groupR3c (No. 167)
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)[2]
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

The name "corundum" is derived from the Tamil-Dravidian word kurundam (ruby-sapphire) (appearing in Sanskrit as kuruvinda).[8]

Because of corundum's hardness (pure corundum is defined to have 9.0 on the Mohs scale), it can scratch almost all other minerals. It is commonly used as an abrasive on sandpaper and on large tools used in machining metals, plastics, and wood. Emery, a variety of corundum with no value as a gemstone, is commonly used as an abrasive. It is a black granular form of corundum, in which the mineral is intimately mixed with magnetite, hematite, or hercynite.[6]

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.[9]

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