content: Some minerals are the product of the radioactive decay of other minerals. For instance in really old rocks it's probably going to be uranium turning into lead. In a mineral where you can safely assume that any trace of one element is a product of the radioactive decay of another element you can simply take a sample of it, measure the ratio of the elements and determine its age to a phenomenally high degree of accuracy. For uranium lead dating - the accuracy is higher than 99%. And it doesn't have to be anything like pure uranium. Various minerals contain trace amounts of radioactive elements. Zircon and Uranium for instance. But that trace is far more than enough for dating.