Simultaneous X-Radiography, Phase-Contrast and Darkfield Imaging to Separate Natural from Cultured Pearls

Abstract

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.

Neutron Radiography and Tomography: A New Approach to Visualize the Internal Structures of Pearls

“…Fissures appear dark, as they are voids with no attenuation effect on X-rays. (Note: Readers wishing to see X-ray phase contrast and darkfield images of this pearl are directed to Figure 6 in Krzemnicki et al, 2017. ) Neutron radiography of this pearl (Figure 3c) shows the inverse to X-radiography, with a brighter core owing to the enrichment of organic matter (i.e.…”

“…However, the fuzzy appearance of the neutron radiograph obtained with the current analytical setup demonstrates that it is not the first choice for visualizing such fine internal features crucial for pearl identification. Much more detailed X-ray phase contrast and darkfield images of this same cultured pearl are presented in Figure 9 of Krzemnicki et al (2017). Figure 7 shows the complex external and internal structures of non-beaded saltwater cultured pearl CP-2m (Pinctada margaritifera; see also Figure 10 in Krzemnicki et al, 2017).…”

“…differentiating between their natural or cultured formation; Figure 1) has been a crucial task for gemmologists, given the enormous differences in their rarity and price. Pearl identification is mostly based on the interpretation of internal structures, traditionally visualized by imaging methods such as X-ray radiography (Anderson, 1932;Scarratt et al, 2000;Strack, 2006;Sturman, 2009) and, more recently, X-ray computed microtomography (Wehrmeister et al, 2008;Karampelas et al, 2010;Krzemnicki et al, 2010;Otter et al, 2014) and X-ray phase contrast and darkfield imaging (Revol et al, 2016;Krzemnicki et al, 2017).…”

“…This paper is part of a larger joint research study between author CSH, the SSEF Swiss Gemmological Institute and the Paul Scherrer Institute, in which a total of 32 samples (13 natural and 19 cultured pearls) were investigated with the aim of comparing various imaging methods for pearl testing (Hanser, 2015). It is a follow-up to the article about X-ray phase contrast and darkfield imaging that was recently published in The Journal (Krzemnicki et al, 2017).…”

Digital Three-Dimensional Imaging Techniques Provide New Analytical Pathways for Malacological Research

“…This modality involves technical adjustments to attenuation contrast-based SRμCT beamlines (Zehbe et al 2010, Saccomano et al 2018, but lately also to lab-based μCT equipment (Zanette et al 2014). In molluscs, this approach has so far only been used to analyze pearls (Krzemnicki et al 2017), but is bound to develop into a promising technique for malacological research in the near future.…”