Which decorative gems and stones may be naturally radioactive?

I just looked into this and found it fascinating. There are really two topics that you have hit upon:

1. Naturally radioactive gemstones

These are minerals that contain radioactive primordial nuclides, such as potassium-40 or thorium-232. Here's a link to a nice page listing many radioactive gems with pictures and the reason for the radioactivity. These could potentially be detected with a Geiger counter, just like how the radioactivity of potassium chloride salt can be measured with a Geiger counter.

2. Irradiated gemstones

It turns out that some gems, when irradiated with ionizing radiation, will change color. This happens because the ionization causes atoms in the crystal lattice to reconfigure, and these crystal defects can be enough to change the color of the gem. This process can be used industrially to create gems that have colors that are rarely found in nature. The gems are not radioactive after this treatment because the type of radiation (gammas or betas) cannot induce radioactivity. Wikipedia has a good article on gemstone irradiation. It turns out that fluorite is one such mineral — it changes color when irradiated with enough ionizing radiation (betas and gammas, in particular). So fluorite could be used as a dosimeter because of this property. Here's a webpage with pictures of gems that the author has colored using irradiation, including fluorite. Though exposed to radiation, the fluorite will not be radioactive. Mark [BRAWM Team Member]