Irradiation

Gemstone irradiation covers a wide range of treatments where gems are exposed to electromagnetic radiation (or, alternatively, subatomic particle bombardment). This does not just mean using gamma radiation (as would be expected), but can also include x-rays, ultraviolet radiation, visible light and even infra-red (so, heating of gemstones could be classed as a type of irradiation).

A nuclear reactor is needed for neutron bombardment of gemstones, a particle accelerator for electron bombardment, and gamma rays are used in a facility that handles radioactive cobalt-60.

The first artificially irradiated gemstone was made by the English chemist Sir William Crookes in 1905 by placing a diamond in radium bromide powder for 16 months. The originally colourless stone became green, but maintained a high degree of residual radioactivity.

Modern gemstones must adhere to strict levels for allowable residual radioactivity for them to be allowed to be sold. Even those at the top end of the legal limit are still very safe - a study done by the US Nuclear Regulatory Commission found that a person wearing a blue topaz stone at the highest level of radioactivity allowed for distribution under their regulations would receive an annual dose of 0.03 millirem. By contrast, a chest X-ray is about 60 millirem.

Table 1. Common irradiated gems (after Ashburgh III 1988)

Gemstone Original colour Treated colour Beryl Colourless Yellow or blue Beryl Blue Green Diamond Colourless or yellow to brown Green to blue Fluorite Colourless Various Pearl Light colours Grey to black or grey-blue Quartz Colourless Brown to black Topaz Yellow to orange More intense yellow/orange Topaz Colourless to pale blue Brown/blue/green Tourmaline Colourless to pale colours Yellow/brown/pink/red/green-red Tourmaline Blue Purple Zircon Colourless Brown to red

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