Corundum is a crystalline form of aluminium oxide (Al 2O 3) typically containing traces of iron , vanadium -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. Corundum has two primary gem varieties: ruby. Rubies are red due to the presence of chromium , and sapphires exhibit a range of colors depending on what transition metal is present. 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
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 and oxygen