Corundum

Corundum is a crystalline form of aluminium oxide typically containing traces of iron, titanium, vanadium and chromium. It is a rock-forming mineral. It is a naturally transparent material, but can have different colors when impurities are present. These impurities are the presence of transition metals in the crystal structure of corundum. Corundum has two primary gem varieties, ruby and sapphire. Rubies are red due to the presence of chromium. 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 derived from the Tamil word Kurundam, which originates from the Sanskrit word Kuruvinda meaning ruby.

Because of corundum’s hardness (pure corundum 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 machines used in machining metals, plastics, and wood.