Measure isotopes for radiometric dating
In other words, the chance that a given atom will decay is constant over time.
Decay rates are measured in half-lives the amount of time in which half of a radioactive element will decay.
Because lead (the stable daughter of uranium) has a very different arrangement of electrons, it does not make its way into the crystal as it is forming.
The formation of crystals in the magma marks the moment that the radio-isotopic clock starts ticking.
Geologists hunt for these particular sorts of rock to date the volcanic eruption in which the rock formed.
For example, as shown at left below, uranium-235 has a half-life of 704 million years.
That means that in 704 million years, one gram of uranium will be reduced to ½ gram of uranium.
These zircon crystals are tiny just a tenth of a millimeter long but they are the key to uranium-lead dating.
If these crystals were pure, they would contain just zirconium, silica, and oxygen; however, uranium happens to have a similar arrangement of outer electrons to zirconium, and so as zircons form, "mistakes" are sometimes made, and uranium is substituted for zirconium.