The quantity that helps determine the absolute age of rocks and fossils is the ratio of carbon-12 to carbon-14.
Carbon-14 dating, also known as radiocarbon dating, compares the ratio of carbon-14 (a radioactive isotope) to carbon-12 (a stable isotope) in organic materials. By measuring this ratio and understanding the half-life of carbon-14, scientists can estimate the time since the organism died, thus determining the age of the sample.