Comparison of Synthetic Colourless Sapphire and Natural Light-Coloured Sapphire

Recently Czochralski synthetic colourless corundum crystals can grow as heavy as one hundred kilograms.Such corundum crystal almost does not contain any inclusion as commonly seen in coloured synthetic ones with curved striae and elongated bubbles.The crystal can be cut and used as facet，which is similar to the appearance of natural colourless or lightcoloured sapphires.By far，such crystals have not yet appeared on the jewellery market. However，it is likely these synthetic or similar products will appear soon.By studying features between synthetic colourless sapphires and natural light-coloured ones，the authors find it is difficult to distinguish them by conventional gemmological methods and UV-Vis-NIR spectrum.Cathodoluminescence results show weak identification features.In terms of the non-destructive identification，visible inclusions are key features for separating the natural light-coloured sapphires from the synthetic colourless ones.Almost all the former ones have natural inclusions such as crystal inclusions and healing fluid inclusions，whereas the latter ones do not have any inclusions. However, if a colourless one is flawless, distinction features between them are difficult to be observed, In this case, results of infrared absorption spectrum, Diamondview^TM and cathodoluminescence seem to be useful tools for identification. Gem inspectors should be alert when meeting light-coloured sapphire without any visible inclusion.