GIA Thailand :: Gemstone Gathering 7th





		Gemstone Gathering The 7th Gemstone Gathering will take place on October 24th, 2007 about " The Identification of Rare Pearls and Pearl Treatments " By Mr. Ken Scarratt Director of GIA Research Thailand This presentation begins by describing a current project where GIA is assisting the discoverers of a significant pearl find. Blue Water Ventures of Key West, Florida, in searching the sunken wreck of the Santa Margarita (which was lost in the year 1622) uncovered a lead box that was found to contain thousands of natural pearls. The Santa Margarita, one of a fleet of ships that sailed the Caribbean calling at ports in Panama, Honduras and Mexico in search of “riches” to adorn the wealthy of Europe, was one of many that fell foul of the hurricane seasons common to that region. The discovery is significant both from historic and a gemological perspectives. Of interest gemologically is that the most likely producing mollusk for these natural pearls is Pinctada imbricata, and as such providing an extraordinarily rare opportunity to examine pearls from a historically significant origin. The presentation summed-up the preliminary gemological results of the investigation with detailed images one of the pearls, the UV/visible reflectance spectrum and chemistry. The latter showed that Pb had been transferred from the box to the pearl. The presentation then described two very unusual discoveries of pearl-encrusted-shells. These discoveries were displayed in the form of detailed microradiographs that revealed in the first case a distinct scallop shell at the center of a button shaped natural pearl from Pinctada maxima, and the second showed the shell of a gastropod, also at the enter of a natural pearl. These two examples, while being exceedingly rare, exemplify why one should not be pedantic about the origins (or reasons for formation) of natural pearls. Moving then to the core subject of the presentation a wide of variety natural pearls from a myriad of pearl producing mollusks were described. Data presented included, color, appearance and surface structures as well as UV/visible, Raman spectra and chemical analyses. While identification criteria that established the producing mollusk for many of the pearls were clearly established for others it was a work-in-progress. The theme statement behind this presentation being “understanding pearls and their growth is key to the development of identification criteria for those that are uncommonly encountered”. The pearls and pearl producing mollusks described included; Crassostrea virginica Pinctada mararitifera Pinctada maxima Pleuroploca gigantea Pleuroploca trapezium Lopha cristagalli Nautilus pompilius Lambis truncata Isognomon attenuata. Atrina vexillium Pinna nobilis Tridacna gigas Strombus gigas Melo aethiopica Melo amphora Melo broderipii Melo georginae Melo melo Codakia tigerina Mercinaria mercinaria Nodipecten nodosus Placopecten magellanicus Periglypta (Venus) magnifica Oliva bulbiformis Cypraea tigis Haliotis dohrniana Haliotis asinina Chicoreus ramosus Cymatium pileare Fusinus undatus Littorina littorea Modiolus philippinarum Mytilus edulis edulis Spondylus regius Spondylus versicolor Chamidae It was emphasized that those described represented a mere snapshot of the potential possibilities. The presentation then moved to treatments applied to pearls and the procedures used to identify these. Beginning with the common and easily identified treatments; firstly dyeing by various methods, including silver nitrate, was described and through images how such treatments could be identified via the observation of color concentrations on the surface of pearls, particularly in the areas of blemishes and the entrances of drill holes. It was shown how silver nitrate dyeing could be determined through the “reversal” of growth structures on a microradiograph or through chemical analysis. Both natural and cultured pearls may contain large voids which may be treated by the filling of these areas with anything from wax to metal balls or wire. These treatments were revealed in the microradiographic images of the individual pearls. A popular color for cultured pearls today is brown – often referred to as “chocolate”. While such colors do occur naturally they are quite rare and therefore the majority of the cultured pearls described as “chocolate pearls” in the market today are a product of a treatment process. One of these treatments is described as “bleaching” and the other is by dying with “silver nitrate”. The appearance of these treated pearls and the treatment identification procedures were presented with the key criteria being linked with, UV/visible spectroscopy, chemistry and UV fluorescence. A new (experimental) treatment was then described whereby some pearls are coated with a thin transparent mixture of aluminum oxide and titanium dioxide. The layers being from between 0.3µm to 3µm they are said to be either chemical or wear resistant dependant on the formula used. Identification criteria presented included chemical analyses and fluorescence, although it was emphasized that the coatings currently examined were not visible under normal gemological microscope magnifications. The presentation then moved to the spectroscopic identification of treatments applied to golden (yellow) cultured pearls, these including UV/visible and Raman data. In conclusion two of the most topical treatments were presented and discussed, these being the bleaching to white of cultured pearls and luster enhancement. Identification criteria for both were briefly discussed. A good evening was had by all participants and GIA Thailand looks forward to seeing all at the next Gathering in November. The title is "Diamond Cutting Facilities in New York" on Thursday, November 29, 2007 at GIA Thailand Campus.