Gemological Characteristics and Coloration Mechanism of Vanadium-Bearing Beryl from Nigeria

Vanadium-bearing beryl is a vanadium-bearing variety of green beryl (distinct from emerald) that exhibits an “electro-optical” green (blue-green) color, which has led to its commercial popularity. However, the underlying coloration mechanism remains unclear. The present study adopted standard gemological tests and non-destructive spectroscopic tests, such as X-ray fluorescence, UV-visible-near infrared (UV-Vis-NIR), infrared and Raman spectroscopy, to analyze the vanadium-bearing beryl from Nigeria. The results of these tests indicated the presence of Fe as the predominant chromogenic element of vanadium-bearing beryl, followed by V, at a level exceeding that of Cr. Furthermore, the samples displayed lower levels of alkali and magnesium when compared to other beryls, accompanied by lower refractive indices and specific gravities. Spectroscopic analysis indicates that the structural channels are dominated by type I H2O, with CO2, HDO, and D2O molecules also present. The inclusions observed in vanadium-bearing beryl bear a resemblance to those found in typical aquamarines, which are raindrop-shaped inclusions, and to those found in emeralds of various origins, which are irregular, jagged, gas–liquid two-phase/three-phase inclusions. The broad UV-Vis-NIR absorption bands at 427 and 610 nm are characteristic of V3+ (and a minor amount of Cr3+). Charge transfer between Fe2+ and Fe3+ may also contribute to the 610 nm band, which is superimposed on the absorption bands of V3+ and Cr3+. These factors primarily contribute to the blue-green coloration of beryl. The absorption induced by V3+ in the visible violet-blue region exhibits stronger intensity and a greater tendency towards the blue region compared to Cr3+. Consequently, the resultant vanadium-bearing beryl acquires the yellow-green hue (induced by V) overlaid with the light blue (induced by charge transfer between Fe2+-Fe3+ pairs), resulting in the so-called “electro-optical” green (blue-green) beryl.