New Insights into the Coloration Mechanism in Spessartines and the Impact of Munsell Neutral Grey Backgrounds

In this study about spessartines,13 spessartine samples with proper conditions are used for FTIR spectrum and UV-Vis spectrum tests to find the gemological characteristics and coloration mechanism. In the UV-Vis spectrum, spessartines’ special color has a relationship with the absorption bands at about 460 nm and 480 nm. Based on the CIE 1976 L*a*b* colour system, we come to the conclusion that both color coordinates a* and b* control the value of chroma C*, and color coordinate b* mostly controls the hue angle h°. We also explore how the different chromogenic ions FeO_tot and MnO and their ratio FeO_tot/MnO influence spessartines’ color, finding that the color of spessartine samples is influenced by both Mn and Fe. By analyzing the FTIR spectrum, we discovered that as the content of Mn decreases, and the A, C and D peaks move to the position of a longer wave, the color of spessartine samples also changes significantly. By using the standard light source D65, we find that N9.5 Neutral Grey Background is the best background to grade the color of spessartines.