Chromogenic Mechanism and Chromaticity Study of Brazilian Aquamarine

Aquamarine, a popular variety of blue beryl, faces challenges in market valuation due to its reliance on subjective color assessment. This study investigates the coloration mechanism and establish a quantitative framework for assessing its color based on spectral and chromaticity analysis. We utilized electron probe microanalysis, ultraviolet-visible-near-infrared spectroscopy, laser Raman spectroscopy, and fiber optic spectroscopy to examine Brazilian aquamarine samples with varying blue intensities. The results indicate that the samples have high alkali metal (Na, K) content and low V/Cr content, consistent with the characteristics of high-alkali beryl. Ultraviolet spectroscopy reveals that the Fe3+-Fe2+ interaction (absorption at 620 nm) is the primary cause of blue coloration, while in deep blue samples, absorption at 956 nm decreases. Raman shifts (317 cm−1, 392 cm−1 Al-O bonds) correlate with TFeO content and chromaticity b value higher TFeO content corresponds to smaller Al–O peak shifts, and larger shifts are associated with higher b values (yellow hue). Specifically, increasing TFeO content leads to a shift of the Al-O Raman peak towards higher wavenumbers, and the magnitude of this shift is negatively correlated with the TFeO level. Based on hue angle (H) and saturation (S), we propose a classification method: “Light Blue” (H: 140–170, S ≤ 15), “Sky Blue” (H: 170–200, 15 < S ≤ 25), “Ocean Blue” (H: 200–230, 25 < S ≤ 35), and “Deep Blue” (H > 230, S > 35). This system provides a scientific basis for the quality assessment and market valuation of aquamarine.