Radiogenic dating methods
Carbon-14 is present a level of about 1 part per trillion in the atmosphere--for every trillion particles of carbon one is C-14.
While this is an extremely trace amount, over time it is incorporated into the systems of most life forms.
Potassium-Argon (K/Ar) dating is a method that applies directly the dating of rocks. Uranium-Lead (U/Pb) dating is one of the oldest methods of radiometric dating, Rutherford being one of the first scientists to note it.