Dating uranium 235
Radioisotopic dating is a key tool for studying the timing of both Earth's and life's history.
This suite of techniques allows scientists to figure out the dates that ancient rock strata were laid down and hence, provides information about geologic processes, as well as evolutionary processes that acted upon the organisms preserved as fossils in interleaved strata.
Thus, you would calculate that your rock is about a billion years old.
These zircon crystals are tiny just a tenth of a millimeter long but they are the key to uranium-lead dating.
If these crystals were pure, they would contain just zirconium, silica, and oxygen; however, uranium happens to have a similar arrangement of outer electrons to zirconium, and so as zircons form, "mistakes" are sometimes made, and uranium is substituted for zirconium.
In other words, the chance that a given atom will decay is constant over time.
Decay rates are measured in half-lives the amount of time in which half of a radioactive element will decay.