Descendants of uranium 238
The radioactive offsprings of uranium and thorium are partly responsible for a fraction of natural radioactivity. Traces of uranium descendants can be found in uranium ores, with 0.34 grams of radium and 0.0000012 milligrams of polonium found in every ton of uranium. The family tree of uranium 238 stops with lead 206, which is a stable and non-radioactive isotope, as opposed to the preceding thirteen products, all of which have equal activities. It was by observing that two different uranium minerals, pitchblende and chalcolite, had higher activities than uranium itself, that Pierre and Marie Curie were able to discover the tiny quantities of polonium and radium they contained.