Radio-thermoluminescent potential of calcareous shell material is a function of CaCO 3 polymorphism. Calcite fossils have a high capacity for thermoluminescent energy storage. Aragonitic shells are inert with respect to radio-thermoluminescence. Aragonite, however, emits luminescence when it reverts to calcite at approximately 425 degrees C. This light is believed to be a triboelectric discharge associated with the sudden reconstructive phase change. The shells belonging to a given taxonomic category tend to have similar and sometimes diagnostic thermoluminescent properties. Thermoluminescent correlation and age-dating of limestones must consider the consequences of biomineralogy and its accompanying thermoluminescence.