So: The general approach to assessing gain or loss is to look at the isotope abundances in different minerals and see if there's a pattern.
If the ratio is constant, we can be pretty sure there's been no gain or loss.
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Crystallization of a mineral is a good way to close a system. Any disturbance of the system effectively resets the clock to zero by allowing decay products to escape or reshuffling the abundances of elements.
Weathering and metamorphism are the two most common ways to disturb a system.
Furthermore, Parentium and Daughterium are so different in chemical properties that they don't otherwise occur together.
If there were such a pair of isotopes, radiometric dating would be very simple.
Suppose, in repaving your driveway, you find a stash of old coins buried in the ground. Of course there are more outlandish explanations, like somebody counterfeiting 1920 coins in 1900 (and successfully anticipating any changes in design in the meantime), or secretly tearing up part of the driveway after 1950, but unless someone comes up with really persuasive evidence, we're justified in ignoring these hypotheses.
The driveway was poured in 1950, and the coins are all dated 1920. Radiometric dating generally requires that a system be closed - in other words, has not had material added or removed.
So accurate determinations require very pure samples, very accurate and selective detectors, or both.
The true age of a sample is self-explanatory, but unless the material dates from historic times, the true age is rarely known.
If you don't have minerals with those elements, you can't date the rock.