Pearls of Pinctada margaritifera, P. maxima and P. radiata.

A) Natural pearls (P. radiata): radiography of a necklace and a cross-section of a pearl showing the three layers: the periostracum rich in organic material (OM) (inner layer), the prismatic layer (middle layer), and the aragonitic nacre or mother of pearl layer (outer layer). B) Beadless (without a nucleus) cultured pearls also called ‘Keshi’ (P. maxima): radiography of a necklace and a cross-section showing the nacreous layer around an inner cavity filled with OM. C) Beaded cultured pearls: radiography of a necklace with P. margaritifera pearls and cross section of an Akoya pearl showing the nacreous layer around an internal nucleus and an OM “pocket” on the right (Photos and radiographies A–C: H.A. Hänni). D) Necklaces with P. margaritifera pearls (lower row left), P. radiata pearls (upper row) and P. maxima pearls (lower row right). The inset shows the historic natural pearl “the Peregrina” which was found in the 16th century. This pearl and its necklace were sold for $11.8 million at a Christie's auction in December 2011 in New York. The PCR-RFLP method described here could provide scientific validation of the provenance of historic pearls (Photos: Swiss Gemmological Institute SSEF). E) Scanning electron microscope side-view image of aragonite tablets of the nacreous layer of a P. margaritifera pearl (Photo: Marcel Düggelin, ZMB, Basel University).