Although Rubidium and Strontium rarely occur in concentrations greater than a few hundred parts per million, they can be found in most rocks.

When a rock forms, it will contain rubidium-87, strontium-87 and strontium-86: as long as nothing is gained or lost from the rock, the amount of rubidium will decrease, and the rate of strontium-87 will increase. Since strontium-86 is not radioactive, its concentration will remain constant.
By measuring the change in concentration of rubidium-87 and strontium-87, it is possible to calculate the age of the rock by using the decay constant.