Ruby and sapphire both belong to the mineral family corundum. Corundum was first synthesised back in 1837 by the French chemist Marc Gaudin, but it was Auguste Verneuil, in the late 19th century, who perfected the process. He invented the flame fusion process, the first commercially successful method of producing synthetic rubies and, later, synthetic sapphires.
Flame fusion process
The flame fusion process, also called the Verneuil process, is also used to manufacture other simulant gemstones. It involves melting a finely powdered substance using an oxyhydrogen flame and crystallising the melted droplets into a crystal boule, as illustrated above. In the case of corundum, the fine powder is aluminium oxide, to which trace amounts of other chemicals can be added to produce the desired colour. From the boule, the gemstones are cut and they will be of much larger size and much purer form than can normally be found in nature.
There are however several tell tale signs that might be present to give away corundum manufactured in this way. As the boule grows upwards, a series of semicircular growth striations appear within it. This curved banding is usually then present in the cut gemstone and can be found using magnification. You can make out this banding in the photograph above. There may also be trapped gas bubbles within the stone. These will be from excess oxygen in the flame creating a flame temperature that is too hot. This causes a localized boiling on the surface of the boule resulting in the capture of gas bubbles. Be aware, however, that many of the other tests used to identify corundum will be correct when applied to Verneuil synthetics, such as the refractometer, polariscope and CCF filter.
Although the Flame fusion process is by far the most common method of producing synthetic corundum, there are two other methods worthy of mention. These are the flux melt growth process and the hydrothermal growth process, both of which are explained with reference to emerald in the blog, Synthetic Emerald.
If you have any rubies or sapphires that you would like to sell, please contact Simon Rufus, gemmologist at Grand Auctions, Folkestone, Kent for further details.
- Posted by Simon Rufus