"The sample I have representing plutonium is the naturally occurring mineral muromontite, which is a mixture of uranium and beryllium. Putting beryllium near uranium is generally considered a bad idea because the alpha particles from the decay of uranium are captured by the beryllium atoms, which in turn release neutrons. Neutrons are very unhealthy to be around.periodictable.com
In the case of this sample, however, the neutrons are in turn re-captured by the uranium, which then undergoes further decay and is transformed into plutonium. The result is that this mineral contains the highest known naturally occurring concentration of plutonium."