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.