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Irradiated Blue Topaz Gemstones

According to Professor Jill Banfield of UC Berkeley, Topaz is the most common irradiated gem on the market. This paper states that virtually all of the faceted blue topaz has been irradiated. While irradiation in an electron beam linear accelerator is relatively safe, topaz which has been bombarded by neutrons in a nuclear reactor (another method used) can become highly radiative, due to activation of impurities in the topaz.

The neutrons are captured by atoms that make up the topaz. This transmutes them to a different isotope, and some of these isotopes are radiactive. Many of them have short half lives, but some have rather long half lives. The three most common are:

  • Scandium-46 with a 84 day half life. It does a beta decay, with emission of two gamma energies, 889 and 1120 keV.

  • Tantalum-182 with a 115 day half life. It also has a beta decay, and emits gamma rays at a variety of energies, roughly betweem 60 to 1230 keV.

  • Manganese-54 with a 312 day half life. It emits gammas of 835 keV.

These all emit highly penetrating gamma rays. For example, an inch of steel would only stop about half of the gamma rays at an energy of 1230 keV.

The term half life means the amount of time that has to pass for half of the radioactive material to decay. That means that after one half life, the amount of radiation produced would be half of the original amount. After a second half life, it would be down to one quarter, and so on.

This means two things. First, the gemstone never becomes entirely non-radioactive, some of the artificially induced radiation will always remain, although eventually it will be reduced to such a low level that it is not above normal background radiation levels. Second, several half lives are required to reach this point. For example, it would take four half lives for the radiation to be reduced to around 6% of the original value. As you can see from the list above, the half lives of the three main isotopes in question are in the range of several months to almost one year. So it could take several years for the stones to reach a relatively normal radiation level.

In addition to artifically induced radiation, some green zircon, as well as other gemstones, are radioactive due to naturally occuring uranium, thorium, or other materials in the stone.

The Nuclear Regulatory Commission (NRC) has requirements governing the import and distribution of neutron-irradiated gems.

There is also a large amount of material concerning blue topaz on this website.

Our GM-10 and GM-45 radiation detectors can detect low levels of radiation, and can be useful in identifying gemstones which have above normal levels of radiation, which should then be examined and tested by a qualified individual or organization.

Thinking of buying a surplus CDV-700 or 715 detector? Be sure to read our report first.