Corundum is a mineral. It is very hard, with a Mohs hardness of 9 (diamond is 10). It is usually clear. Its chemical formula is aluminium oxide, Al2O3. Sometimes some of the aluminium atoms get replaced with chromium atoms. It then becomes a ruby. Other impurities can make a sapphire.
Quick facts General, Category ...
| Corundum |
|---|
 |
|
| Category | Oxide mineral – Hematite group |
|---|
Formula
(repeating unit) | Aluminium oxide, Al
2O
3 |
|---|
| Strunz classification | 04.CB.05 |
|---|
| Dana classification | 4.3.1.1 |
|---|
| Crystal symmetry | Trigonal (32/m) |
|---|
| Unit cell | a = 4.75 Å, c = 12.982 Å; Z=6 |
|---|
|
| 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 |
|---|
| 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 light |
|---|
| References | [2][3][4][5] |
|---|
|
| Sapphire | Any color except red |
|---|
| Ruby | Red |
|---|
| Emery | Black granular corundum intimately mixed with magnetite, hematite, or hercynite |
|---|
Close
