Etude du changement de couleur des perles par traitement

This study demonstrates that natural colors of freshwater cultured pearls from mollusks of the Hyriopsis genus are due to colored mixtures of different pigments and not to a single pigment. This is a mixture of unsubstituted polyenes, not carotenoids, with the general chemical formula R-(-CH=CH-)n-R’. A mixture of pigments from the same family is observed in colored pearls and/or the shells, from: Strombus gigas, Melo melo, Mytilus edulis, Mercenaria mercenaria, Cypraea cervus, Busycon carica, Pleuroploca gigantea, Swiftopecten swiftii, Neodipecten nodosus, Margaritifera margaritifera, Pinctada fucata, Pinctada albina and Pinctada radiata. Pearls from the bivalve Pinna are the only pearls found until today with colors due to a mixture of carotenoids. Colored pearls and/or shells from Pinctada margaritifera, Pinctada maculata, Crassostrea virginica, Lopha cristagalli, Pinctada mazatlanica, P. fucata and Pteria penguin contain a variety of porphyrins. In some samples it was not possible to identify the exact pigments but only their spectroscopic characteristics. The yellow color of pearls from P. maxima, P. fucata και P. Margaritifera is due to the reduction in reflectance from 330 to 460nm, with peaks at about 355 and 435 nm. Different colors of pearls from P. margaritifera are the result of a mixture of pigments with absorptions at 405, 435, 460, 495, 530, 585, 625, 650, 700 and 745 nm in the visible spectrum. The majority of treated-color samples show that treatments may decompose the pearls’ organic matter. Hence, their Raman spectra show broad bands at about 1350 and 1600 cm-1. Moreover, their reflectance spectra show differences in shape and relative intensities of peaks with center at 280 nm compared to the natural-colored samples. Additionally, some treated-color pearls show Raman peaks that have never observed in the natural-colored samples. Distinction between treated- and natural-color pearls strongly depends on the pearls’ mollusk. In some cases a combination of classical and advanced non-destructive methods are necessary.Στην εργασία αυτή δείχνεται ότι τα φυσικά χρώματα των καλλιεργημένων μαργαριταριών γλυκού νερού από Hyriopsis δεν οφείλονται σε μία χρωστική ουσία αλλά σε μίξη περισσοτέρων. Οι χρωστικές αυτές δεν ανήκουν στην οικογένεια των καροτενοειδών, αλλά περιέχουν ‘απλές πολυενικές αλυσίδες’ στο μόριό τους και έχουν γενικό χημικό τύπο R-(-CH=CH-)n-R’. Μίγμα ‘απλών πολυενικών ουσιών’ περιέχονται στα χρωματιστά μαργαριτάρια ή/και στα κελύφη από: Strombus gigas, Melo melo, Mytilus edulis, Mercenaria mercenaria, Cypraea cervus, Busycon carica, Pleuroploca gigantea, Swiftopecten swiftii, Neodipecten nodosus, Margaritifera margaritifera, P. fucata, P. albina και P. radiata. Τα μαργαριτάρια από δίθυρα γένους Pinna είναι τα μοναδικά που περιέχουν μίγμα καροτενοειδών ουσιών. Τα μαργαριτάρια από P. margaritifera, P. maculata, Crassostrea virginica, Lopha cristagalli, P. mazatlanica, P. fucata και Pteria penguin περιέχουν ένα είδος πορφυρίνης. Σε κάποια δείγματα δεν κατέστη δυνατό να αναγνωρισθεί οι ακριβείς χρωστικές ουσίες αλλά μόνο τα φασματικά τους χαρακτηριστικά. Το χρώμα των κίτρινων μαργαριταριών από P. maxima, P. fucata και P. margaritifera οφείλεται σε μείωση της ανακλαστικότητας από τα 330 ως τα 460 nm, με μέγιστα περίπου στα 355 και 435 nm. Τα χρώματα των μαργαριταριών από P. margaritifera οφείλονται σε συνδυασμό χρωστικών ουσιών που δίνουν απορροφήσεις στο ορατό στα: 405, 435, 460, 495, 530, 585, 625, 650, 700 και 745 nm. Τα περισσότερα δείγματα επεξεργασμένου χρώματος έδειξαν ότι η επεξεργασία προκαλεί αποσύνθεση της οργανικής ουσίας. Στα φάσματα Raman αυτών παρατηρούνται κορυφές περίπου στα 1350 και 1600 cm-1. Ακόμα, στα φάσματα ανάκλασης τους, σε σύγκριση με τα φυσικού χρώματος δείγματα, πολλές φορές οι κορυφές με κέντρο τα 280 nm παρουσιάζουν διαφορετικές σχετικές εντάσεις και σχήματα. Επίσης, σε κάποια τεχνητού χρώματος μαργαριτάρια παρατηρούνται κορυφές Raman που δεν παρατηρούνται στα φυσικού χρώματος δείγματα. Σε συνάρτηση με το μαλάκιο προέλευσης του φυσικού χρώματος μαργαριταριού, η αναγνώριση τεχνητού ή φυσικού χρώματος απαιτεί πολλές φορές συνδυασμό απλών γεμολογικών και επιτηδευμένων μη καταστρεπτικών μεθόδων

Raman spectra of gem-quality variscite and metavariscite

International audienc

Mineralogy and Geochemistry of Gems

Gems have been used in the manufacture of jewellery and as ornaments since antiquity. Considering gems, recent statistics have shown that about 15 billion Euros are annually at stake. Nowadays, gemmology, i.e., the study of gem materials, is one of the most expanding fields in the earth sciences, positioned between academia and industry. As an applied science, in gemmology, the instruments used should be non- or microdestructive, and their cost should be reasonable both in terms of equipment and time consumption. Gemmology can also be used contribute to the development of pure science and in some cases, destructive techniques may have to be used. Taking into account the fact that gems are albeit rarely available for scientific research, this compilation of 20 articles by around 100 researchers from over 30 different institutions situated in 20 countries from around the globe, presented in the Special Issue entitled “Mineralogy and Geochemistry of Gems”, offers very good examples on the application of various methods for their study which will hopefully contribute to our better understanding of gem formation in general and will enhance scientific debates attracting more scientists from various disciplines to get involved in this field

Editorial for Special Issue “Mineralogy and Geochemistry of Gems”

Gems are materials used for adornment or decoration that must satisfy several criteria where they must be aesthetic and visually appealing; relatively rare; hard and tough enough to resist “normal” wear; and able to withstand corrosion by skin contact and cosmetics [...

Hyalite with Magnificent Internal Features

International audienc

UV-VIS-NIR Reflectance Spectroscopy of Natural-Color Saltwater Cultured Pearls from Pinctada Margaritifera

International audienceNatural-color saltwater cultured pearls from Pinctada margaritifera were studied by diffuse reflectance UV-Vis-NIR spectroscopy to identify the absorption features associated with their various colors. Nine patterns observed in the visible range demonstrated that individual colors are caused not by one pigment but by a mixture of pigments

Identification of the Endangered Pink-to-Red Stylaster Corals by Raman Spectroscopy

International audienceAll corals within the Stylasteridae family (including the Stylaster genus) are listed in Appendix II of CITES; this means they are protected and their trade requires an export permit, unlike corals from the Corallium genus, which include most pink-to-red corals used in jewelry. Raman scattering demonstrates that corals from the Stylaster genus contain carotenoid pigments (polyenic pigments substituted with methyl groups), whereas those from the Corallium genus are colored by unmethylated polyenic pigments. Additionally, Stylaster corals are made of aragonite, whereas those from Corallium are composed of calcite. Through Raman scattering analysis, the fully protected Stylaster pink-to-red corals may be distinguished from this other type of gem coral

Spectral Differentiation of Natural-Color Saltwater Cultured Pearls from Pinctada Margaritifera and Pteria Sterna

International audienc