Abstract
Two groups of commercial quality (“acceptable”) pearls produced using two donors, and a group of “acceptable” pearls from other donors were analyzed using reflectance UV-Vis spectrophotometry. Three pearls with different colors produced by the same donor showed different absorption spectra. Cream and gold colored pearls showed a wide absorption from 320 to about 460 nm, while there was just slight reflectance around 400 nm by the white pearl with a pink overtone. Cream and gold pearls reached a reflectance peak at 560 to 590 nm, while the white pearl with pink overtone showed slightly wider absorption in this region. Both cream and gold pearls showed an absorption peak after the reflectance peak, at about 700 nm for the cream pearl and 750 nm for the gold pearl. Two other pearls produced by the same donor (white with cream overtone and cream with various overtones) showed similar spectra, which differed in their intensity. One of these pearls had very high lustre and its spectrum showed a much higher percentage reflectance than the second pearl with inferior lustre. This result may indicate that reflectance is a useful quantitative indicator of pearl lustre. The spectra of two white pearls resulting from different donors with the same color nacre (silver) showed a reflectance at 260 nm, followed by absorption at 280 nm and another reflectance peak at 340 nm. After this peak the spectra for these pearls remained flat until a slight absorption peak around 700 nm. Throughout the visible region, all white pearls used in this study showed similar reflectance spectra although there were differences in reflectance intensity. Unlike the spectral results from white pearls, the results from yellow and gold pearls varied according to color saturation of the pearl. The results of this study show that similarities between absorption and reflectance spectra of cultured pearls resulting from the same saibo donor are negligible and could not be detected with UV-Vis spectrophotometry. Nevertheless, this technique could have a role to play in developing less subjective methods of assessing pearl quality and in further studies of the relationships between pearl quality and that of the donor and recipient oysters.
Highlights
The process of cultured marine pearl production requires a piece of nacre-secreting mantle tissue from a donor oyster, to be grafted with a round nucleus into the gonad of a recipient pearl oyster
Spectral analysis of pearls originating from two donors, represented by the upper and lower graphs in Figure 1, respectively, showed the same reflectance peak recorded in the UV region at 260 nm, followed by absorptions ranging from 270 to 280 nm, a second peak in the region of 320 to 340 nm
P. maxima did not [13], allowing treated pearls to be distinguished from natural pearls of similar color
Summary
The process of cultured marine pearl production requires a piece of nacre-secreting mantle tissue from a donor oyster (known as saibo), to be grafted with a round nucleus into the gonad of a recipient pearl oyster. The grafted mantle tissue proliferates to form a layer of secretory tissue, which deposits successive layers of nacre onto the nucleus This process results in the formation of a cultured pearl after approximately two years [1]. UV-visible (UV-Vis) spectrophotometry utilizing a diffuse reflectance accessory can be used to assess material properties based on reflected light. UV-Vis spectrophotometry has been used to record the reflectance of pearls from Pinctada maxima and P. margaritifera [13,14] as well as freshwater pearls [15] These studies showed that pearl color could be clearly characterized by peaks in reflectance spectra, which are correlated with the presence of particular pigments in the nacre layers [13,14]. Emphasis was placed on the application of this procedure in assessing pearls produced by the same mantle donors and pearls of various colors (from white/silver to gold) and overtones
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