The 235U–207Pb cascade has a half-life of 704 million years and the 238U–206Pb cascade is considerably slower, with a half-life of 4.47 billion years.
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Third, zircon is widespread in igneous rocks as a primary mineral.
This makes it especially valuable for dating these rocks, which have no fossils to indicate their age.
Fourth, zircon is physically tough and easily separated from crushed rock samples because of its high density.
Other minerals sometimes used for uranium-lead dating include monazite, titanite and two other zirconium minerals, baddeleyite and zirconolite.
However, zircon is so overwhelming a favorite that geologists often just refer to "zircon dating."But even the best geologic methods are imperfect.
Dating a rock involves uranium-lead measurements on many zircons, then assessing the quality of the data.
Uranium easily substitutes for zirconium while lead is strongly excluded.
This means the clock is truly set at zero when zircon forms.
Some zircons are obviously disturbed and can be ignored, while other cases are harder to judge.