In this study, X-ray phase contrast imaging with a grating interferometer is applied on pearls for the first time in order to distinguish natural pearls from cultured pearls. Traditionally, this separation is mainly based on X-ray radiography. In order to visualize the internal structure of pearls we used a custom-made grating interferometer setup and performed measurements on three different pearl products, a natural pearl, a beaded cultured pearl and a beadless cultured pearl. To enhance the visibility of the internal pearl structures, we applied a high-pass filter in order to better conclude on the applicability of this technique to the separation of natural and cultured pearls. The study shows that it is possible to visualize internal pearl structures using distinctly shorter exposure times compared to traditional X-ray radiography and that X-ray phase contrast imaging is a promising complementary method for pearl analysis.

The separation of natural from cultured pearls is mainly based on the interpretation of their internal structures, which traditionally have been visualized by X-radiography and more recently by X-ray computed microtomography (micro-CT). In this study, the authors present a new analytical approach using a grating interferometer, which simultaneously generates an X-radiograph, a phase-contrast image and a small-scale scattering or darkfield image. The latter two additional images provided by this technique offer detailed and complementary information, as they are especially sensitive for visualizing tiny material inhomogeneities in pearls such as fissures, organic layers and cavity structures. Using seven selected natural and cultured pearl samples and a strand of non-beaded freshwater cultured pearls, the authors demonstrate that this new analytical approach offers versatile and rapid pearl identification possibilities, especially as it is possible to analyse not only single loose pearls but also entire strands and necklaces. Compared to micro-focus digital radiography and micro-CT, certain limitations in resolution still remain with the described prototype setups, and as such this new methodology should be considered a helpful complementary technique to the classical radiography of pearls.

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Non-destructive imaging of the internal structures of pearls has so far been mainly based on X-ray imaging methods. As organic matter is almost transparent to X-rays, the identification of some structures can be difficult. This study shows that neutron imaging can be a helpful complementary method to visualize structures inside pearls beyond standard X-ray radiography and tomography, as neutrons are highly attenuated by hydrogen-bearing (organic) matter within pearls. The use of neutron radiography and tomography is shown for selected natural and cultured pearls (beaded and non-beaded). In addition, we present neutron phase contrast and darkfield images of a beaded cultured pearl, in analogy to X-ray phase contrast and darkfield imaging described in a previous study. While neutron imaging of pearls is particularly useful for understanding material inhomogeneities and void structures, this methodology is currently only available at large-scale facilities that are equipped to deal with nuclear reactions.