A relative dating method used for bone. Calcium ions in the phosphatic mineral hydroxyapatite are gradually altered after burial into uranium ions as a result of uranium being in solution in the percolating groundwater. The longer bone has been in the ground, the more uranium will have been absorbed. The local environmental conditions affect the rate and therefore there is no universal rate which yields absolute dates. The uranium is radioactive and the emitted beta particles are measured using a Geiger counter. The method is much less destructive than the chemical analyses required for the nitrogen and fluorine tests.