The different colors found within members of the corundum group arc due to small amounts of metal oxide impurities. Corundum without impurities (and therefore without color) is rare, but when found is classified as colorless sapphire. Stones made up of different colors, including colorless areas, arc more common. Stones like these are generally orientated by the cutter so that the color is at the base. Then, when viewed from above, color fills the stone.

Truly colorless sapphire is found in Sri Lanka. (Cloudy or milky colored sapphire is also found in Sri Lanka, and referred to locally as geuda. Heat treatment of geuda produces blue sapphire, much of which is faceted and used in jewellery. Some Sri Lankan corundum shows red, blue, and colorless areas, which may be faceted or polished to give an interesting stone.

Synthetic colorless corundum has been produced by the Vermeil Method since about the 1920s, and has been called diamonded.