The farther back we go, the less we know. The first abundant fossil evidence does not appear until the beginning of the Cambrian period. Because Precambrian rocks are old they have been subjected to a great many changes, and many of the clues present in the original rocks have been destroyed. There are difficulties in dating the geologic time scale in general, and sedimentary sequences in particular. Not all rocks can be dated by radiometric techniques.
Igneous rocks can - the age is the time when minerals crystallized.