Gemmological Investigations on Pearls and Emeralds using Neutron Imaging

Gemmology deals with the characterization of coloured stones, diamonds and pearls used in the jewellery sector. As the investigated objects are in general rather valuable, a large variety of non-destructive testing methods (e.g. X-ray luminescence, X-ray tomography, UV/VIS spectroscopy, etc.) is routinely used for their inspection and characterisation.
In a joint project of Paul Scherrer Institut (PSI), Swiss Gemmological Institute (SSEF) and the University Freiburg, potential application fields of neutron imaging methods (i.e. radiography, microtomography and neutron grating interferometry) in the characterisation and testing of pearls and emeralds were investigated and compared to already established X-ray methods. Neutron tomography yields results with comparable image quality but a different contrast, highlighting in the case of pearls the regions containing organic and hence hydrogen containing material. As such regions composed of low-Z material can be very hard to distinguish from voids inside an object using X-ray tomography, neutron tomography provides important additional information on the tested object due to its complementary properties. The complementarity between neutron and X-ray data shows also in the case of emeralds, where fissures filled with organic fillers are highlighted in the neutron data, while staying concealed in the X-ray data. Metallic inclusions in the emeralds on the other hand appear much more pronounced in the X-ray data, then using neutron imaging. The utilization of both methods on the same sample yields hence additional information on the composition of different regions within the object.