Irradiation has the ability to act on the optical properties of gemstones, like the color: it can intensify or alter it. Thanks to the presence of natural radiation, this process can naturally take place over a long period of time. Certain natural gemstone colors, such as blue-to-green colors in diamonds, are the result of the exposure to natural radiation, in particular alpha and beta-radiation. The limited penetrating ability of these particles results in partial coloring of the diamond's surface. Artificial high-energy radiation can speed up the process and it can produce fully saturated colors, increasing their commercial value. Indeed, gemstone irradiation processing is widely accepted in the jewelry industry in some states. Despite this, the practice is not accepted worldwide. Irradiated gemstones are internationally traded and are for sale, even in those states in which the practice is prohibited.

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Figure 1 - Natural topaz and color enhanced.

Topaz is the most commonly treated stone: typically it is yellow-orange and after radiation exposure it becomes blue. Diamonds and other precious gems may also be treated with radiation. Diamonds are usually irradiated to become yellow, blue-green or green, while pearls to become gray or black. There are three different methods to irradiate gemstones: gamma irradiation, irradiation with an electron beam and neutron irradiation. Sometimes the treatment is also combined with heating that allows more colors to be obtained.
Gamma ray treatment is the safest and cheapest

irradiation method, but it is not widely used because the complete treatment takes several months. In diamonds, the color produced is a blue to blue-green which penetrates the whole stone. Such diamonds are not heated.
Electron beam irradiation is one of the most common processes which, for diamonds, produces a blue, blue-green, or green color but it only penetrates about 1 millimeter deep. The process can be combined with heating at temperatures of 500–1200 °C producing orange, yellow, brown, or pink colors.
The other common treatment is neutron irradiation, that penetrates the entire stone and produces a green to black color. By heating at temperatures from 500–900 °C an orange, yellow, brown or pink color can be obtained. Blue to blue-green stones are not heated.

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Figure 2 - Colorless and irradiated, in some cases combined with heating, diamonds.

ut irradiation with e-beams and neutrons in some gems, unlike gamma rays, could lead to the formation of radionuclides, called activationproducts: gems become radioactive. In general, the longer the stones are exposed to radiation, the deeper and more attractive the color, but even higher is the possibility that the trace elements or impurities in the stone are

activated. The activation products are an undesired by-product of the irradiation process. Unlike most other consumer products, for which radionuclides are specifically added to perform important functions, in the case of gemstones they serve no useful purpose. However, the half-life of these activation products is usually short, up to a few weeks: they decay very quickly. Before being distributed, the treated gemstones are set aside for a couple of months in order to allow any radioactivity to decay. The storage of the gemstones for an appropriate period of time after irradiation can ensure that most of the activation products decay to insignificant levels before the sale. Only a limited number of activation products have longer half-lives, among which 54Mn (half-life 312 days) and 134Cs (half-life 752 days). However, after storage, the activity of the gemstones is always evaluated because they cannot be provided to the public unless they comply with the exemption criteria of the regulatory authorities.
The distribution of irradiated gemstones is well regulated by authorities requesting the producers to analyze the radioactivity before placing the gemstones on the market. Subsequent distributors, jewelers or other retailers are exempt from regulations. At the time of sale, the gemstones have an increased activity compared with their state prior to irradiation, but the associated individual doses are low. Of course, we are not in danger: the dose imparted from these gems would be extremely small and it would continue to decay becoming consequently lower and lower over time.
Because intense colors do not occur naturally, such as the blue colors in topaz or strong green, pink, and red colors in diamonds, such stones are considered to have undergone irradiation treatment. Determination whether a diamond is naturally colored or not requires examination by an experienced gem-testing laboratory.