Some isotopes of elements undergo radioactive decay. These atoms spontaneously disintegrate by emitting atomic particles such as helium nuclei, electrons) and thus, transform to different elements.
Example: Isotopes of Uranium decay to lead (Pb).
U 238 -> Pb 206 + 8He
U 235 -> Pb 207 + 7He
The rate of decay is known, so that measurement of isotopic ratios can be used to determine the age of formation of minerals.
Emitted particles heat and damage the surrounding mineral. This can cause development of non-crystalline regions within crystals as well as small defects (color centers) in the mineral and surrounding minerals.