Great question! In a nutshell, it's because we choose minerals whose crystal structure can only bond with the original element and not with it's "daughter" tha

Great question! In a nutshell, it's because we choose minerals whose crystal structure can only bond with the original element and not with it's "daughter" that's produced after radioactive decay. For example, Uranium/Lead dating of zircons is one of the most common methods (particularly for very old rocks). There's a reason we use zircons for uranium/lead dating, and not just any mineral that contains trace amounts uranium. In zircon (ZrSiO4), uranium can (and does, in small amounts) substitute for zirconium in the crystal structure and bond with the silica tetrahedra. However, lead cannot bond with the silica tetrahedra in zirconium's place. Therefore, the zircon crystal structure "rejects" lead, and any lead in the crystal lattice at the present day must be the result of radioactive decay. If we know the rate of radioactive decay of the uranium isotopes in question (there are two uranium isotopes that decay to two different lead isotopes), all we need to date the rock is the present-day ratio of a particular uranium isotope to the corresponding lead isotope. In practice, you also exploit the fact that there are two uranium isotopes that independently decay to two different lead isotopes. This gives two independent dates, and disagreements between them can be use to test for contamination, etc. (There are also a few other checks I won't get into... I'm not a geochronologist, and each different isotopic system and instrument used to measure it has slightly different protocols. Reality is messy, but this gives the basic picture.)