Many zircon crystals contains trace amounts of uranium and thorium. These radioactive elements were incorporated into the zircon at the time of crystallization. They convert into their decay products at a steady rate. The ratio of parent materials to daughter products can be used to estimate the time of crystallization. Using this method, the oldest mineral grains in the world are zircon crystals found in Australia. They are estimated to be about 4.4 billion years old.

When radioactive elements in zircon crystals or nearby materials decay, radiation is emitted. The zircon crystal can be damaged by this radiation. Some zircon has been so damaged by exposure to this radiation that it no longer retains the clarity and optical properties of an attractive gem material. This is why some zircon is not suitable for use as a gem.