Radiometric dating
The method works best if neither the parent nuclide nor the daughter product enters or leaves the material after its formation. Anything which changes the relative amounts of the two isotopes (original and daughter) must be noted, and avoided if possible. Contamination from outside, or the loss of isotopes at any time from the rock's original formation, would change the result. It is therefore essential to have as much information as possible about the material being dated and to check for possible signs of alteration.