Isotopes are important to geologists because each radioactive element decays at a constant rate, which is unique to that element.These rates of decay are known, so if you can measure the proportion of parent and daughter isotopes in rocks now, you can calculate when the rocks were formed.Furthermore, Parentium and Daughterium are so different in chemical properties that they don't otherwise occur together.

For example, fission track dating measures the microscopic marks left in crystals by subatomic particles from decaying isotopes.

Another example is luminescence dating, which measures the energy from radioactive decay that is trapped inside nearby crystals.

These use radioactive minerals in rocks as geological clocks.

The atoms of some chemical elements have different forms, called isotopes.

We could be sure that a mineral containing parentium originally had no daughterium.

If the mineral contained 1 part per million Parentium-123 and 3 parts per million Daughterium-123, we could be sure all the Daughterium-123 was originally Parentium-123.

Relative dating and radiometric dating are used to determine age of fossils and geologic features, but with different methods.

Relative dating uses observation of location within rock layers, while radiometric dating uses data from the decay of radioactive substances within an object.

These break down over time in a process scientists call radioactive decay.