Is the pearl layer a reversed shell? A re-examination of the theory of pearl formation through physical characterizations of pearl and shell developmental stages in

A series of physical characterization methods (UV fluorescence microscopy, X-ray microdiffraction, backscattered electron imaging and X-ray absorption spectroscopy) were applied to Polynesian pearls collected after different cultivation periods, varying from three weeks to eighteen months. Through this rigorous time-based sampling, 120 pearls produced by 20 different donor oysters were compared. Results show that the structure of the pearl layer can be understood as a sequence of distinct secretion processes whose progressive occurrence through time may lead to variously arranged and sometimes aberrant mineralized structures. By making comparisons with the structure and growth mode of the Pinctada margaritifera shell, this study shows that the currently accepted theory that views the pearl-bed as a “reversed shell” cannot account for the diversity of the microstructural patterns and mineralogical properties observed in the pearl layers. From a practical and economic view point, it appears that development of these pre-nacreous materials superposed onto a perfectly round-shaped nucleus is the main cause of shape irregularities in pearls and the consequent decrease in their value.