Radioactive minerals
Minerals containing thorium, uranium or any of their descendants can be considered to be slightly radioactive, the main exposure resulting from the gamma rays which escape from the rock. Pitchblende and chalcolite, shown above, are two of the minerals which have played an important role in the discovery of radioactivity. Uranium is a very rare element, and uranium ores rarely contain more than a few percent of it. It is present in much smaller amounts in rocks such as granite.