Pinctada Margaritifera Cultured Pearl with Baroque-Shaped Nucleus

G & G January 10, 2013 Volume 4, No. 4

The French Gemological Laboratory (Laboratoire Français de Gemmologie, or LFG) recently analyzed 110 Tahitian black cultured pearls presented as keshi (cultured pearls without a nucleus). They were large, up to 25.5 mm in the longest dimension, and slightly baroque, with a very good nacre quality. Overtones ranged from green (“peacock”) to purple (“aubergine”).

Their UV-Vis-NIR spectrometry features were consistent with cultured pearls from Pinctada margaritifera, particularly the presence of the 700 nm band (S. Karampelas, “UV-Vis-NIR reflectance spectroscopy of natural-color saltwater cultured pearls from Pinctada margaritifera,” Spring 2011 G&G, pp. 31–35). But microradiography did not reveal the expected structures of keshi (see A strong, easily visible delineation ran roughly parallel to the external shape, 0.8–2.8 mm below the surface. That line was similar to the structure encountered in cultured pearls at the boundary between nacre and nucleus, but here it had a “baroque” shape. The shape of the cultured pearl generally followed the shape of the nucleus. In many cases there were one or more cavities related to this structure. In several instances, this delineation opened into cavities, with a very similar appearance to that seen in keshis. X-ray opacity, which appeared as shades of gray in radiography, was very similar between the nucleus and the periphery, which is usually the case for cultured pearls.
We obtained permission to cut one of the submitted pieces in half to directly observe the type of core used for nucleation. The material forming the nucleus was similar to that normally seen in cultured pearls, but the shape was different. Raman analysis indicated that the nucleus was indeed composed of calcium carbonate (aragonite). The yellow X-ray luminescence and the Sr/Mn ratio determined by EDXRF chemical analysis (Rigaku NexCG) confirmed that it was produced by a freshwater mollusk, not Pinctada margaritifera. When one of the two halves was cut again, the core and black nacre separated immediately, revealing another human intervention: a polished surface on the nucleus.
This appears to be a new type of core carved from a piece of shell from a freshwater mussel (e.g., Megalonaias sp., Fusconaia sp., or Quadrula sp.) conventionally used to create spherical nuclei but here carved with a baroque shape. The quality of the nacre used for the baroque nuclei seems lower than that used for spherical beads, however. Indeed, this lower quality induces many structures such as cracks, fissures, and areas of variable X-ray opacity that are visible in radiographs. These structures may lead one to believe that this is not a nucleus, as it looks more natural.
Cultured pearls with spherical pieces of shell as nuclei have been long known, and nucleation with freshwater cultured pearls has appeared more recently. But this new baroque variety of nucleus may be difficult to properly identify using X-radiography because the features produced sometimes resemble those observed in keshis or baroque-shaped natural pearls.
- Olivier Segura
Laboratoire Français de Gemmologie (LFG), Paris
Emmanuel Fritsch
University of Nantes, France