Radioactive dating is a method of dating rocks and minerals using radioactive isotopes.
Radioactive decay is a natural process and comes from the atomic nucleus becoming unstable and releasing bits and pieces.
These are released as radioactive particles (there are many types).
For an element to be useful for geochronology (measuring geological time), the isotope must be reasonably abundant and produce daughter isotopes at a good rate.
Either a whole rock or a single mineral grain can be dated.
The Re-Os isotopic system was first developed in the early 1960s, but recently has been improved for accurate age determinations.