Unique spectral characteristics of natural-color Edison pearls cultured in Hyriopsis cumingii, and its formation mechanisms

Xuejun Yan; Yangming Jiang; Hui Jin; Tao Chen; Yang Zhou; Jinhua Liu; Jun Yan

doi:10.1016/j.micron.2022.103324

Unique spectral characteristics of natural-color Edison pearls cultured in Hyriopsis cumingii, and its formation mechanisms

Micron. 2022 Sep:160:103324. doi: 10.1016/j.micron.2022.103324. Epub 2022 Jul 4.

Authors

Xuejun Yan¹, Yangming Jiang¹, Hui Jin¹, Tao Chen², Yang Zhou³, Jinhua Liu¹, Jun Yan⁴

Affiliations

¹ Zhejiang Fangyuan Test Group Co., Ltd, Hangzhou, Zhejiang 310013, China.
² Gemmological of Institute, China University of Geosciences, Wuhan 430074, Hubei, China.
³ College of Information and Electronic Engineering, Zhejiang University of Science and Technology, Hangzhou, Zhejiang 310023, China.
⁴ Zhejiang Fangyuan Test Group Co., Ltd, Hangzhou, Zhejiang 310013, China. Electronic address: yanj_zjut@163.com.

PMID: 35816928
DOI: 10.1016/j.micron.2022.103324

Abstract

Using ultraviolet-visible-near infrared (UV-Vis-NIR) diffuse reflectance spectroscopy, Raman spectroscopy, and field emission scanning electron microscope (FE-SEM), we comparatively investigated the unique spectral characteristics of different natural-color Edison pearls. Edison pearls were firstly divided into two types based on theirs corresponding reflection spectral characteristics: type A with basically uniform reflection feature for each sample; and type B with non-uniform reflection feature for each one. Raman spectroscopy revealed that complex organic pigments with bands at about 1015, 1124-1134, 1297, 1507-1526, 2245, 2620, and 3019 cm^-1, gave rise to the colors of Edison pearls. Interestingly, two bands in the Raman spectra of dark-colored pearls, at about 1124-1134 cm^-1 and 1507-1526 cm^-1, corresponded to the stretching modes of the C-C single bond (ν₂) and the CC double bond (ν₁), respectively, shifted significantly toward low wavenumbers. Meanwhile, the intensity ratio between the two bands (I₁₁₂₄-₁₁₃₄/I₁₀₈₅) increased as pearl color became more saturated. In addition, six bands at about 2139, 2417, 2521, 2976, 3332, and 3702 cm^-1 were present in the Raman spectra of all dark-colored samples, while these bands were absent in the Raman spectra of light-colored ones. This indicated that the exact organic pigments in Edison pearls were not completely consistent, which led to variation in the coloration and the dominant absorption band in the reflection spectra of pearl. More importantly, based on our SEM results, as well as our UV-Vis reflectance spectroscopic analysis, we first proposed that the positions and relative intensities of the reflection peaks located between 320 nm and 600 nm were likely to be associated with the density and the degree of regularity of grooves appeared on the surface of pearl. These findings substantially enhance our knowledge of gemological characteristics of colored Edison pearl and the formation mechanism of unique reflection features of them. Furthermore, this study revealed key characteristics that can be used to identify imitation and color-treated Edison pearls.

Keywords: Colored Edison pearl; Diffuse reflectance spectroscopy; Groove; Organic pigment; Raman spectroscopy; Surface morphology.

Publication types

Research Support, Non-U.S. Gov't

MeSH terms

Coloring Agents*
Spectrum Analysis, Raman*

Substances

Coloring Agents