Treatments & Synthetics
Synthetic
Star Sapphires
One should be extremely careful while identifying star gemstones, as synthetic stars are very inexpensive to produce and are common. Synthetic stars appear to be of high quality, with very little or no irregularities and extremely sharp stars. Natural star gems tend to be irregular and imperfect, and the stars may be wavy, whereas the synthetic stars have straight lined stars. Synthetic sapphire can be cheaply produced in the laboratory.
Lindy star sapphires were manufactured en masse by the Lindy division of Union Carbide from the 1950s to the 1970s. Since then artificial star sapphires have been made in Southeast Asia. A Lindy (sometimes spelled Linde) has an ‘L’ stamped on the bottom of the stone, though other synthetics do not. Synthetic Star Sapphires are currently produced in Japan by Nakazumi Earth Crystals.
A lab-created star sapphire mounted on a sterling silver ring
Star Sapphire Treatments
Star Sapphires are usually subject to Heat Treatment. Before cutting, the stones are heated to a temperature of about 3100-3300 degrees Fahrenheit (i.e. 1700-1800 degrees Celsius) for several hours. Heating of the gem improves tone and saturation of the color. Such treated stones are worth less than an untreated gem of similar appearance. A reputable dealer should always tell if a stone has been subject to heating.
The other type of treatment performed on star sapphires is the Diffusion treatment which exposes the surface of the stone to certain chemicals combined with heat. The diffusion is done on the surface of the star sapphire and could extend to around a millimeter into the stone. That process supports the star effect on the gemstone. The hardness of the gemstone is not affected by the diffusion treatment.
It is usually difficult to find a natural star sapphire which has not been subject to any treatment. However, if you do wish to buy such a stone, you may have to pay a small premium for it.
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Star Sapphires