Spectral Characteristics and Color Origin of Unstable Yellow Sapphire
WANG Yu-yan1, YANG Ling-yue1, LI Ming2, YANG Peng-tao3, Andy Hsitien Shen1, WANG Chao-wen1*
1. Gemmological Institute, China University of Geosciences (Wuhan), Wuhan 430074, China
2. Jewelry Institute, Guangzhou Panyu Polytechnic, Guangzhou 511483, China
3. Earth Sciences, China University of Geosciences (Wuhan), Wuhan 430074, China
Abstract：Yellow sapphires with unstable color are widely available in the market. How to effectively identify the characteristics of color unstable yellow sapphires is a hot topic in gemological research. In this paper, color-changing experiment, Ultraviolet-visible spectroscopy and 3D fluorescence spectroscopy were employed to study spectral characteristics of color-stable yellow sapphires. The color-changing experiment revealed that a part of yellow sapphires from Sri Lanka have unstable color, showing a photochromic phenomenon. Short-wave ultraviolet light dyed the original color, while sunlight faded the original color. The color of yellow sapphire after ultraviolet light is composed of stable part and unstable part. Color-unstable sapphire in the “colored state” and the “faded state” both showed blue-violet region absorption in the UV-Vis spectrum, which was likely related to the charge transfer of O2--Fe3+, causing the stable yellow coloration of sapphire. The UV-Vis spectrum of sapphire with obvious color changes showed a significant absorption enhancement in the blue-violet region in the “colored state” compared to the “faded state”, which might attribute to ultraviolet irradiation enhancing the charge transfer between O2--Fe3+. The UV-Vis spectroscopy result showed that samples had a weak Fe-related absorption peak, consistent with low Fe content. The low Fe concentration in the yellow sapphire is not enough to produce a stable yellow in color. 3D fluorescence spectra results revealed that the samples in “colored state” and “faded state” had the same fluorescence center (excitation wavelength=325～335 nm and emission wavelength=560～570 nm). The fluorescence intensity in the “colored state” is much higher than that in the “faded state”. Iron-containing yellow sapphire has a fluorescent effect, and the characteristic fluorescent center can be used as a potential identification characteristic to identify color-unstable yellow sapphire. This paper comprehensively reports the spectral characteristics and possible color causes of unstable yellow sapphires, providing potential identification characteristic to identify color-unstable yellow sapphire and a theoretical basis for the subsequent color modification process.
王雨嫣，杨凌岳，李 明，杨鹏涛，沈锡田，王朝文. 颜色不稳定黄色蓝宝石的谱学特征与颜色成因[J]. 光谱学与光谱分析, 2021, 41(08): 2611-2617.
WANG Yu-yan, YANG Ling-yue, LI Ming, YANG Peng-tao, Andy Hsitien Shen, WANG Chao-wen. Spectral Characteristics and Color Origin of Unstable Yellow Sapphire. SPECTROSCOPY AND SPECTRAL ANALYSIS, 2021, 41(08): 2611-2617.
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