The key is to measure an isotope that has had time to decay a measurable amount, but not so much as to only leave a trace remaining.Given isotopes are useful for dating over a range from a fraction of their half life to about four or five times their half life.This is frequently because the selected technique is used outside of its appropriate range, for example on very recent lavas. The Institute for Creation Research's RATE project aimed to show scientifically that methods of radiometric dating produced wildly inconsistent and incorrect values.
Radiometric dating — through processes similar to those outlined in the example problem above — frequently reveals that rocks, fossils, etc.
Through analysis, a bone fragment is determined to contain 13% of its original carbon-14.
The half-life of carbon-14 is approximately 5,730 years. Since the quantity represents 13% (or 13/100ths) of , it follows that Thus the bone is approximately 17,000 years old.
Note that although carbon-14 dating receives a lot of attention, since it can give information about the relatively recent past, it is rarely used in geology (and almost never used to date fossils).
Carbon-14 decays almost completely within 100,000 years of the organism dying, and many fossils and rock strata are hundreds of times older than that.