The various colours throughout members from the corundum group result from small quantities of metal oxide impurities. Corundum without impurities (and for that reason without colour) is rare, however when found is classified as colourless sapphire. Stones comprised of different colours, including colourless areas, tend to be more common. Stones like thee are usually orientated through the cutter so the colour reaches the bottom. Then, when viewed from above, colour fills the stone.
Truly colourless sapphire can be found in Sri Lanka. Cloudy or milky coloured sapphire can also be present in Sri Lanka, and known locally as “geuda”. Heat treating geuda produces blue sapphire, high of that is faceted and utilized in jewellery. Some Sri Lankan corundum shows red, blue and colourless areas, which can be faceted or polished to provide a fascinating stone.
Synthetic colourless corundum may be made by the Verneuil method since concerning the 1920’s, and it has been called diamondite.
