text: Uranium-lead dating is usually performed on the mineral zircon (ZrSiO), though other materials can be used.  Zircon incorporates uranium atoms into its crystalline structure as substitutes for zirconium, but strongly rejects lead. It has a high blocking temperature, is resistant to mechanical weathering and is chemically inert.  Zircon also forms multiple crystal layers during metamorphic events, which each may record an isotopic age of the event. These can be dated by a SHRIMP ion microprobe.
  One of the advantages of this method is that any sample provides two clocks, one based on uranium-235s decay to lead-207 with a half-life of about 703 million years, and one based on uranium-238s decay to lead-206 with a half-life of about 4.5 billion years, providing a built-in crosscheck that allows accurate determination of the age of the sample even if some of the lead has been lost.