If the rocks have an interbedded lava flow or volcanic ash bed, it's gold.
The older our sample is, the more daughter isotope it will contain relative to the parent.
What radioactive materials actually do is decay according to a law: Decays/Time = K * Number of atoms K is a constant called the decay constant.
Let t stand for time and N(t) stand for the number of atoms at time t .
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.
In particular, quartzites and carbonate rocks almost always don't have enough to permit dating.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.In other words there was originally 4 parts per million Parentium-123 and 0 parts per million Daughterium-123.Since there is now only 1/4 of the original amount of Parentium-123, we know that two half-lives of Parentium-123 have elapsed.