The new method for identify golden-lipped pearls

Abstract

1. Background
Golden-lipped pearls have become increasingly popular due to their rarity and gorgeous appearance. However, in recent years, colored pears that elaborately imitate golden-lipped pearls have come into the market and it is an urgent task to develop methods for identificating these colored pearls from uncolored pearls. Conventional identification methods that have been used for golden-lipped pearls are the coloring mark observation method, the ultraviolet irradiation florescence observation method and the spectral reflection method. With the ultraviolet irradiation florescence observation method, it has been reported that colored pearls emit florescence and uncolored pearls ("natural pearls") emit only weak florescence. It has been suggested that the reason for natural pearls emitting weak fluorescence is that deep color natural pearls contain more yellow pigment in the nacreous layer, which blocks protein fluorescence. It has been reported that with the spectral reflection method, natural golden-lipped pearls show characteristic absorbances at 280 nm, 360 nm and 450 nm. However, due to the recent improvement of coloring techniques, coloring marks cannot be detected around holes anymore, and there are many cases where colored pearls do not emit fluorescence, making difference between natural and colored pearls difficult. In this study, as a new highly accurate non-destructive method for golden-lipped pearls, Raman spectroscopy was investigated.
2. Materials and methods
Colored and natural golden-lipped pearls were compared using the ultraviolet irradiation fluorescence observation method, the spectral reflection method and the Raman spectroscopy. Additionally, the pearl samples that were suggested by the results to be natural were cut into slices to determine if they were colored or uncolored pearls.
3. Results and discussion
With the ultraviolet irradiation fluorescence observation method, some colored pearls were observed to emit fluorescence (e.g., red or blue and white color under short-wave or long-wave ultraviolet), whereas other colored pearls did not emit fluorescence just like natural pearls do not; it was therefore suggested that the identification would be difficult with this method.
With the spectral reflection method, one or more of the three absorbances characteristic to natural golden-lipped pearls were not observed or shifted in colored pearls. A problem with this method is that pearls are influenced by reflection interference colors because pearls are multilayer spheres. When a pearl is set in the integrating sphere of a spectroscopic analyzer for measurement, normally the body color of the surface is measured; however, if teri is strong, measurements are strongly influenced by reflection interference colors. Therefore, when analyzing pearls with strong teri, natural pearls may lose their characteristic absorbances, resulting in their being misjudged as colored pearls.
On the other hand, with the Raman spectroscopy, significant differences were found between colored pearls and natural pearls, with the former showing the scattering intensity of 5,000 or higher and the latter showing the scattering intensity of 5,000 or lower, suggesting that this method can be used for identificating the two purposes. Also, because some colored pearl samples showed similar spectral data to natural pearls, the slices of the colored pearls were observed under an optical microscope. The observation confirmed no colored layer on the outer layer, confirming that they were natural pearls. Therefore, it was suggested that Raman spectroscopy was a highly reliable non-destructive method.