What can be dated?

In order for any material to be radiometrically dated it must have incorporated radioactive isotopes within its crystal structure. Because they are the right size some  radioactive parent isotopes are capable of being incorporated into some mineral crystals. However, because the daughter isotopes are a different size they are excluded from the crystal structure. This means that at the time of crystallization the mineral will only contain parent isotopes, in other words at the time of crystallization the mineral contains 100% parent isotope and 0% daughter isotope. As time goes by daughter isotopes are unable to escape the solid crystal. Thus, radiometric dating of a mineral will yield the time of crystallization.

Except under rare circumstance sedimentary rock cannot be radiometrically dated. This is due to the fact that sedimentary rock is produced through the compaction and cementing of other rock fragments. Since no part of the lithification process would reset the 'atomic clock' within individual minerals any date measured would represent the time the individual mineral grain crystallized not the time the sedimentary rock was lithified. The only exception to this occurs when the mineral glauconite is present within the rock. This mineral forms in marine environments as a result of chemical reactions with clay minerals during lithification. Because glauconite incorporates radioactive Potassium 40 into its crystal structure it can be used to date the time of lithification.

Metamorphic rock can also be radiometrically dated due to the fact that the extreme pressures and temperatures required to metamorphose rock change the crystal structure of individual minerals which allow daughter isotopes to escape the previously formed mineral. This affectively resets the parent/ daughter ratio and allows geologists to radiometrically date the time the metamorphism occurred. 

Living, well dead, organism can also be radiometrically dated. In this processes scientist compare the ratio of Carbon 12, a stable isotope of carbon, to that of Carbon 14, the radioactive isotope. Both Carbon 12 and Carbon 14 occur naturally within our environment at a constant ratio (although carbon 14 is constantly decaying new carbon 14 is produced in the upper atmosphere as Nitrogen 14 is bombarded by cosmic radiation). Because Carbon 12 and Carbon 14 exists in a constant ratio within our environment it also exist in the exact same ratio within living organism. This is possible due to the fact that living organism are constantly replenishing carbon 14 levels through respiration. However, when an organism dies respiration ceases and carbon 14 levels are no longer replenished. As a result the Carbon12/ Carbon 14 ratio begins to change and we determine the time of death through comparison to the known Carbon 12/ Carbon 14 Ratio