The effects of nacre microstructure on green and pink interference colors in Pinctada fucata martensii pearls

Despite the importance of both pearl body and interference colors in determining pearl quality, only body color has been studied extensively. We report novel information on relationships between pearl nacre microstructure and interference color saturation and hue in pearls produced by Pinctada fucata martensii. The interference color saturation and hue of 60 round pearls was subjectively graded by eye by an expert. Nacre optical characteristics and microstructure of pearls of 1) high (grade A) and low (grade B) interference color saturation, and 2) pink and green interference hues are reported. Using electronic pearl quality measuring devices, grade A pearls have higher interference color values (0.050 ± 0.061) than grade B pearls (0.008 ± 0.039), consistent with subjective grading by eye. SEMs of surface microstructure reveal the nacre tablet area of grade A pearls, 4.079 ± 2.173 μm², to be generally greater than that of grade B pearls, 2.940 ± 1.213 μm² (P < .001), with the nacre surface of grade A pearls smoother than grade B pearls. Regardless of grade, the basal and middle regions of the nacreous layer of pearls with a pink interference color hue differ significantly in nacre tablet thickness (p < .001), but nacre tablet thickness does not differ significantly between their central and surface regions. The nacre tablet thickness of pearls with green interference color hue changes more consistently from the basal to surface regions, with only the basal and surface regions differing significantly in thickness (p < .001). We report pearl interference color to be greatly affected by nacre tablet microstructure. The saturation of pearl interference color is affected by the smoothness of the nacre surface, which is a function of the surface nacre tablet area. The hue of pearl interference color is affected by differing thicknesses of nacre tablets through the nacreous layer, from surficial to basal regions. Results of this study might benefit the pearl industry, particularly where P. fucata is cultured. Additional study on how shell matrix protein genes are related to nacre tablet biomineralization would improve understanding of the mechanisms affecting pearl nacre microstructure, leading to saturation and hue of pearl interference colors.