Thereafter, the parent isotope steadily decays and the daughter isotope remains trapped in the crystal. All you have to do is measure the ratio of the two amounts, look up the half-life of the parent isotope in a physics book, and it is easy to calculate the age of the crystal. As I said earlier, fossils are usually found in sedimentary rocks, while dateable crystals are usually in volcanic rocks, so fossils themselves have to be dated indirectly by looking at volcanic rocks that sandwich their strata.
