It is not used to date rocks or other inorganic material.
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It is widely used in dating fossils or archaeological samples containing organic material such as wood, charcoal, bone, shells, etc.
A sample that is sealed from the surrounding environment is more likely to avoid contamination or loss than one in an open environment where materials may be carried into or out of the sample by water or simple diffusion.
Violations of this assumption can frequently be identified.
Surprisingly, in at least one case the date range given by There may be other examples of systematic variation in isotope decay rates.