The formula for corundum is Al2O3. The corundum structure consists of hexagonal close packed (a
close packing) O atoms, with Al filling up 2/3 of the central octahedral sites. The Al atoms occur two in a row, then
skip a site. This pattern is staggered for adjacent rows. Corundum is found in metamorphic rocks, including in
aluminum-rich limestone near the seashore which has been metamorphosized. This makes it common in rift zones. It is
also found in source regions of a melt with low Si and high Al content. Corundum is one of the primary materials to
crystallize in deep-seated basalts.
It is used in scratch-resistant optics, scratch resistant coatings on expensive watches, spacecraft and satellite
windows (because of its transparency from the UV to IR), and as an abrasive (as emery, a naturally occurring
impure corundum). The most common impurity in corundum is the colorless Ga+3 ion. The color of various varieties
is given in the following table.
| color |
impurity |
name |
use |
| blue (*) |
 ,
 ( ) | sapphire |
natural gemstone |
| colorless |
pure,  | corundum |
natural |
| red |
 (0.1%) |
ruby |
natural gemstone |
| yellow |
 (0.5-1.0%) |
yellow sapphire |
natural gemstone |
| yellow |
 | |
synthetic |
| red |
 | Ti-doped corundum |
synthetic solid state laser |
color the result of intervalence charge transfer
coupled substitution
