On the deep origin of the depressed rings on pearl surface illustrated from Polynesian Pinctada margaritifera (Linnaeus 1758)

Among the various defects that contribute to depreciate the commercial value of pearls, the occurrence of depressed rings is the most spectacular. Through a series of structural, physical and chemical characterizations of the pearl layer, this paper reveals that the origin and initial stages of these essentially superficial defects are to be found in the earliest stages of pearl formation. The disturbance in growth of the nacreous envelopes is the physical cause of the occurrence of these depressed areas. Attention is also drawn on the unexpected relationship between these large morphological alterations of the pearl surfaces and more punctual defects primarily well visible in the strongly coloured Polynesian pearls. An understanding of the actual origin of these very negative patterns opens the way to reduce their statistical occurrence by paying attention to the grafting practice.