Carbon has an atomic number of 6, an atomic weight of 12.011, and has three isotopes: carbon-12, carbon-13, and carbon-14.(The numbers 12, 13 and 14 refer to the total number of protons plus neutrons in the atom's nucleus.First, the older the object, the less carbon-14 there is to measure.Radiocarbon dating is therefore limited to objects that are younger than 50,000 to 60,000 years or so.Conversely, contamination by newer plant matter carried by flowing water or intruding plant roots may result in a date that is much too young. The original technique was based on counting the number of individual radioactive decay events per unit of time, using a device similar to a Geiger counter.Archaeologists are acutely aware of these and other potential difficulties, and take extreme care in the selection and handling of objects to be dated. In the 1970s a new technique was developed called Accelerator-based Mass Spectrometry (AMS), which counts the number of carbon-14 atoms directly.
The most common radioactive element in granite is Uranium-238.
This turns out not to be exactly true, and so there is an inherent error between a raw "radiocarbon date" and the true calendar date.