1. The amount of a radioactive isotope present in the rock. This is the "parent" isotope, the one that decays over time.
2. The amount of the stable daughter product of that radioactive isotope. This is the element that the parent isotope decays into.
By comparing the amounts of the parent and daughter isotopes, scientists can calculate how long the decay process has been taking place, which directly tells them the age of the rock.
