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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.So when a mineral grain forms (specifically, when it first cools below its trapping temperature), it effectively sets the uranium-lead "clock" to zero.Lead atoms created by uranium decay are trapped in the crystal and build up in concentration with time.For inorganic materials, such as rocks containing the radioactive isotope rubidium, the amount of the isotope in the object is compared to the amount of the isotope's decay products (in this case strontium).For organic materials, the comparison is between the current ratio of a radioactive isotope to a stable isotope of the same element and the known ratio of the two isotopes in living organisms.If you took rocks of all ages and plotted their two Pb/U ratios from their two isotope pairs against each other on a graph, the points would form a beautiful line called a concordia (see the example in the right column).

The two cascades are different—235U becomes 207Pb and 238U becomes 206Pb.

Some zircons are obviously disturbed and can be ignored, while other cases are harder to judge.

In these cases, the concordia diagram is a valuable tool.

Radiocarbon dating is one such type of radiometric dating.

A process for determining the age of an object by measuring the amount of a given radioactive material it contains.

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