Radiometric dating techniques take advantage of the natural decay of radioisotopes.An isotope is one of two or more atoms which have the same number of protons in their nuclei, but a different number of neutrons.Nevertheless, the fundamental assumptions employed are not.
Radiometric Dating - A Brief Explanation Radiometric dating is the primary dating scheme employed by scientists to determine the age of the earth.Radioisotopes are unstable isotopes: they spontaneously decay (emitting radiation in the process -- thus making them radioactive).They continue to decay going through various transitional states until they finally reach stability. It will spontaneously decay until it transitions into Lead-206 (Pb206).The numbers 238 and 206 represent these isotopes' atomic mass.
The Uranium-238 radioisotope goes through 13 transitional stages before stabilizing into Lead-206 (U238 Pb206).
(This is consistent with the geologic 'age' assigned to the granites in which these zircons are found.) There is a significant amount of helium from that '1.5 billion years of decay' still inside the zircons.