Absolute radiation dosimetry may be obtained from natural materials using the principles and techniques developed and utilized in medical and radiation physics. The problems peculiar to natural thermoluminescence dosimetry are (1) post-irradiation calibration techniques, (2) dose equivalency factors, and (3) secular equilibrium effects. These problems are illustrated and solutions are suggested by a study of naturally thermoluminescent mollusc shells. The dose equivalence and dose rates measured in these shells agree well with the known radioactivities of these materials and environments. It is suggested, therefore, that the natural radiation dosimetry of post-Pliocene calcites may be used as a basis for relative age and time correlations.