Noninvasive Investigation of Red Gemstones on Devotional Jewelry From Sicily: Focus on Garnet Composition

A group of richly decorated devotional jewelry from Sicily (18th–19th centuries) has been studied noninvasively by portable Raman spectroscopy, with the aim of identifying the gemstones that embellish these ex-voto. Different levels of richness were observed in the various objects, from those displaying rubies and diamonds to those decorated with garnets and quartz. Thanks to the high number of garnets found, a focus could be carried out on this mineral, proposing two methods for the treatment of the Raman spectra, involving also a group of reference garnets. On one hand, a development of the software MIRAGEM, called MIRAGEM+, was tested to establish the composition of the garnets. On the other hand, Principal Component Analysis (PCA) was applied to the whole complex of spectra to check the possibility of distinguishing groups of garnets with different compositions. The combination of the results obtained with the two methods demonstrated that both, coupled to Raman spectroscopy, are reliable tools for the noninvasive characterization of garnets and can be used with a longer pretreatment when few samples are involved (MIRAGEM+) or with a shorter preparation for a higher number of spectra (PCA).