Minerals containing radioactive elements as part of their structure are modified over time by the decay of the radioactive elements. This modification is termed metamictization or metamiction (see http://en.wikipedia.org/wiki/Metamictization). Zircon is one of those minerals that contains sufficient uranium to undergo this process. The photographs below show zircon crystals that have undergone metamictization of their core structure with a resultant expansion that cracked the later growth zircon surrounding the core.