The use of radiation to color gemstones, especially diamonds (blue topaz is also common). Generally, nuclear reactors, linear accelerators, and gamma ray facilities are used to this end.

Green is the most common color sought by irradiation for diamonds simply because natural green diamonds, even in the rough, are nigh impossible to find.

Irradiation is very easy to detect, especially in faceted stones. The radiation only colors the surface of the stone, so if the stone is polished or abraded somehow, the color can sometimes wear off. In addition, in faceted stones, the color from irradiation will be more vivid near the culet since the gem is thinner there.

One of the first tests done to detect irradiation is a test to determine whether the stone is still "hot," or radioactive. Technological advances have helped to prevent radioactive stones from entering the market by allowing effective testing of the gemstones. However, blue topaz typically stays radioactive for a year after being treated, and diamonds can remain hot for several to several thousand years.

For other techniques of enhancing color and/or clarity, see Gem Enhancement.