Summary: Low Ceylon zircons showing an optical absorption band at 5200 Å appear to have suffered more radiation damage than other low Ceylon zircons. Very heavy fission-fragment irradiation failed to lower the density of each of two low Ceylon zircons significantly, although the specimens were rendered completely amorphous. Further fission-fragment irradiation experiments indicated that low Ceylon zircons showing a previously reported anomalous absorption spectrum have been heated late in their geological histories.