Chromatic Coloration Mechanism of Nigerian Blue-Green Beryl: Multi-Ion Synergistic Effects

The blue-green “electro-optical” green beryl from Nigeria has gained increasing commercial attention, yet its specific coloration mechanism remains unclear. This study investigates ten samples using XRF, UV–Vis–NIR, IR, and Raman spectroscopy to characterize their chemical compositions, spectral features, and coloration origins. The chromophoric elements are predominantly Fe, Cr, and V, with Fe significantly higher than Cr and V. UV–Vis–NIR spectra reveal that the blue-green hue results from synergistic absorptions of V3+/Cr3+ in the yellow-green region, Fe2+–Fe3+ intervalence charge transfer (IVCT) in the red region. Chromaticity analysis shows centroid color differences ΔE*ab of 7.31 between “V-bearing“ and “V not detected” groups, and 20.39 between “high-TFeO/(Cr2O3 + V2O5)” and “low-TFeO/(Cr2O3 + V2O5)” groups, both far exceeding the visual perceptibility threshold. This study provides quantitative spectroscopic and chromaticity constraints for understanding the multi-ion synergistic coloration of Nigerian blue-green beryl.