Reports that the (U-Th-Pb) isotopic system in natural zircon will typically have a closure temperature greater than 900 degrees Celsius, which explains why zircon is apparently such a robust geochrometer. Direct measurements of the uranium, thorium and lead loss from a thermally treated gem-quality natural zircon; The use of zircon in determining age, origin and thermal history of rocks by U-Th-Pb geochronology.