I understand well how radiometric dating works in theory.
In practice, I'm not sure how one would calibrate this for rocks. I've always been told that we can determine rocks' formation time via radiometric dating. This implies that something about the radioisotope abundances are known (or assumed) at the time of formation.
Teachers have said that rocks' "internal clocks" reset when they form. Suppose you take a 1 Gya rock, melt it in a furnace, and then let it re-freeze. According to what I've heard, the rock's "internal clock" has now reset, which implies that its radiometric abundances are the same as when it first formed. This doesn't make sense: if, over its 1 Gyr lifetime, it lost 99% of isotope A, it won't suddenly get the original abundances of that isotope back through the process of melting and freezing. How, then, would one be able to distinguish its date of formation?