Attempt to Identify between Akoya pearls and South-sea pearls -1

Abstract

Akoya pearls and silver-lipped pearls both have white-type, yellow-type and blue-type pearls, and methods for differentiating akoya and silver-lipped pearls have been studied for many years but it has not been established. Depending on the shell size and the sea area from which mother shells are harvested, oyster species have been estimated based on pearl size, the thickness of the nacreous layer ("maki"), pearl texture, among other things. In recent years, however, improvement of aquaculture techniques and other factors have enabled the market distribution of akoya pearls sized 10 mm or more and silver-lipped pearls sized 10 mm or less. Therefore, it is an urgent task to establish methods for differentiating akoya pearls and silver-lipped pearls.
Differentiation methods that have so far been reported are the ultraviolet laser excitation emission spectrum method (Komatsu et al., 1984) and the X-ray fluorescence analysis method (Ito, 2002). The former method measures differences in the spectral patterns of ultraviolet laser excitation between akoya and silver-lipped pearls, which are caused by the differences in organic substrates. However, such differences are erased by processes such as bleaching and thus the method has not been put to a practical use. The latter method uses an X-ray fluorescence analyzer to calculate the intensity ratios Sr/Ca and Na/Ca to differentiate akoya and silver-lipped pearls. Since a part of overlapping domain exists, it is not put in practical use.
In this study, by focusing on the differences in constituent elements and organic substrates between the two pearls, the following differentiation methods were performed.
First, using TOF-SIMS (Time of Flight Secondary Ion Mass Spectrometry), a method which measures the elemental composition and chemical structure of the top surface of pearls, the shells and pearls of akoya pearl and silver-lipped pearl oysters were analyzed. Second, LA-ICP-MS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry) was used to compare the constituent elements between white-type akoya pearls and white-type silver-lipped pearls harvested in a particular sea area.