UV-Vis spectra of the spinels.

Color is the most critical factor in determining the value of gem-quality spinel. This paper examines the color mechanism and the colorimetric characteristics of spinel crystals under D65 and A light sources against nine neutral backgrounds. It also explores color clustering for over 400 spinel crystals in yellow, red, purple, and blue hues. Various analytical techniques were employed, including optical absorption spectroscopy, Energy-Dispersive X-ray Fluorescence analysis, a benchtop sphere spectrophotometer, the Munsell neutral value gray scale chart, and a standard illumination box. The study reveals distinct optical absorption peaks corresponding to different spinel colors, with these peaks being assigned to specific mechanisms based on previous research. Color analysis demonstrates that the chroma and hue of orange spinel crystals are primarily influenced by the red tone under both daylight and incandescent light. Additionally, red, and yellow tones significantly enhance the color of red spinel. For purple spinel, chroma shows a strong correlation with the a* value under both D65 and A light sources, while hue is easily influenced by the blue tone. In blue spinel, chroma is controlled by the green tone, and hue is affected by the blue tone. Lightness and chroma of all spinel colors increase significantly with the luminance factor of the Munsell neutral background, following a power function relationship with varying rates. However, the hue angle remains relatively unchanged, as gray backgrounds have minimal effect on hue. The colors of spinel crystals can be effectively predicted using calculated equations based on the luminance factor of the background under different light sources. The K-means clustering method is particularly effective for categorizing each spinel color into three distinct groups, which is crucial for developing a reliable color grading system for spinel.

颜色是决定宝石级尖晶石（gem-quality spinel）价值的最关键因素。本文以九种中性背景为参照，探究了D65标准照明体（D65 standard illuminant）与A光源（A light source）下尖晶石晶体的呈色机理及色度学特性，并针对黄、红、紫、蓝四色系的400余颗尖晶石晶体开展颜色聚类研究。本研究采用了光吸收光谱法（optical absorption spectroscopy）、能量色散X射线荧光（Energy-Dispersive X-ray Fluorescence）分析、台式积分球分光光度计（benchtop sphere spectrophotometer）、孟塞尔中性值灰度卡（Munsell neutral value gray scale chart）及标准光源箱（standard illumination box）等多种分析手段。研究发现，不同颜色的尖晶石具有各自特征性的光吸收峰，并结合既往研究将这些峰归因于特定的呈色机制。色度分析表明，在日光照明与白炽灯照明条件下，橙色尖晶石的彩度与色相主要受红色调影响；此外，红、黄色调可显著强化红色尖晶石的颜色表现。对于紫色尖晶石，在D65与A光源下，其彩度与CIE LAB色空间的a*分量（a* value）呈强相关性，而色相则易受蓝色调影响。在蓝色尖晶石中，彩度由绿色调主导，色相则受蓝色调调控。所有色系尖晶石的明度与彩度均随孟塞尔中性背景的亮度因子显著提升，二者呈幂函数关系且增长速率各不相同；但色相角基本保持稳定，因灰色背景对色相的影响极小。基于不同光源下背景亮度因子构建的计算方程，可有效预测尖晶石晶体的颜色表现。K均值聚类（K-means clustering）法可将各色系尖晶石精准划分为三个独立组别，该结果对于建立可靠的尖晶石颜色分级体系至关重要。