method of estimating the age of rocks, minerals, and meteorites from measurements of the amount of the stable isotope strontium-87 formed by the decay of the unstable isotope rubidium-87 (rubidium) that was present in the rock at the time of its formation. Rubidium-87 comprises 27.85 percent of the total atomic abundance of rubidium, and of the four isotopes of strontium, only strontium-87 is formed by its decay. The method is applicable to very old rocks because the transformation is extremely slow: the half-life, or time required for half the initial quantity of rubidium-87 to disappear, is approximately 50 billion years.

Most minerals that contain rubidium also have some strontium incorporated when the mineral was formed, so a correction must be made for this initial amount of strontium to obtain the radiogenic increment (i.e., the increase due to decay of rubidium-87).