Heating the crystals, it turns out, liberates these electrons, emitting a measurable amount of light.
Researchers can thus determine the amount of time that has passed since the buried crystal was last exposed to heat.
Indeed, just last month researchers described a fossil that pushes the origins of key mammal features back some 45 million years.
And last week scientists announced that new dates for an extinction event that claimed most of Australia's large animals show that humans, not the climate, wiped them out.
Other radioactive isotopes can be used to accurately date objects far older.
The decay of argon 40 to argon 39, for instance, played a vital role in underscoring the significance of two ancient human skulls unearthed in the Republic of Georgia last summer. Swisher III of the Berkeley Geochronology Center and his colleagues reported, are more than 1.7 million years old, and as such represent the first humans to leave Africa to colonize the rest of the world.
Because of that condition, scientists say, the technique is well suited to dating meteoritic impacts, fire-treated stones used by early humans, cooking hearths and old ceramics.Somewhat similar to thermoluminescence, electron spin resonance (ESR) dates crystals, too (although these are found in shells and enamel.) Unlike thermoluminescence, however, this method counts the number of "unpaired spins" of electrons trapped in the crystal, instead of freeing them.ESR can be used to evaluate materials up to one million years old and has become an indispensable tool for paleoanthropologists, who often use it to date the teeth of animal remains found among the precious human fossils.Some use radioactive isotopes; others take advantage of different phenomena, such as thermoluminescence and electron spin resonance.Still others, like amino acid racemization, show promise but have not yet taken wing.