Irradiation Treatment
Virtually every blue topaz sold in the market today is the product of irradiation treatment. Blue topaz is actually very rare in nature, and tends to a very pale blue. The lovely blue topaz we find today are produced by subjecting clear topaz to large doses of electrons from a linear accelerator or by exposing them to neutrons from a nuclear reactor.
The color in topaz is produced from defects in the crystal structure rather than from chemical elements in the stone. So exposing topaz to radiation induces color changes by modifying the way in which the crystal structure absorbs frequencies of light. Depending on the duration and type of irradiation, and the sort of heating process used afterward, the results vary from sky, to Swiss to London blue. Other slight color variations have been produced and given their own tradenames like "electric blue" and "neon blue". London blue is the scarcest and most expensive type because it requires neutron exposure (the most expensive process), and the longest holding times.
Although the irradiation of gemstones can leave some residual radioactivity, its duration is relatively short. Government agencies in the USA, and other gem irradiating nations, have strict regulations for the holding and testing of irradiated gems to assure that they are not released to the public until they are safe to handle and wear.
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