Using microradiography to assess nacre thickness of mabé pearls: Technique suitability and insights

Nacre thickness is a major factor influencing the quality and value of half-pearls ('mabé'), yet accurate determination of nacre thickness, in a non-destructive manner, is problematic. Microradiography is commonly used to measure the nacre thickness of round pearls, however the suitability of this technique for mabé has not previously been assessed. Mabé were cultured in winged pearl oysters (Pteria penguin) for periods of 200 to 380 days to produce pearls with a range of nacre thicknesses. The nacre thickness of each pearl was quantified using microradiography and standard micrometer techniques. A strong, significant relationship was observed between measurements obtained using both techniques (R2 = 0.88, P ≤ 0.001). Microradiography produced defined, measureable images, with a resolution of ±0.04 mm, for mabé pearls with nacre thicknesses between 0.1 and 1.3 mm. Mabé nacre thickness and pearl quality were significantly influenced by the position of the pearl nucleus within the shell (T(95) = 7.14, P < 0.001, Fisher's exact test, P < 0.001, respectively). Nuclei in a posterior ventral position on the left shell valve produced the highest percentage of good quality mabé pearls (83% A grade or higher) and had a mean nacre thickness nearly twice that of mabé pearls produced at other positions in the shell (0.78 ± 0.04 mm). A significant influence of culture duration on nacre thickness or pearl quality was not detected. Our results demonstrate that microradiography is an accurate and non-destructive technique that is appropriate to determine nacre thickness in mabé pearls. Future research should further examine the relationship between nucleus position and pearl quality to develop optimum culture protocols for mabé pearl production.