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