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Article

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

by
Yunlong Hong
,
Yu Zhang
*,
Xinyi Shao
,
Yanyi Mu
and
Yuemiao Yu
School of Gemmology, China University of Geosciences (Beijing), Beijing 100083, China
*
Author to whom correspondence should be addressed.
Minerals 2025, 15(6), 557; https://doi.org/10.3390/min15060557
Submission received: 15 April 2025 / Revised: 13 May 2025 / Accepted: 21 May 2025 / Published: 23 May 2025
(This article belongs to the Special Issue Formation Study of Gem Deposits)
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Abstract

:
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.

1. Introduction

Beryl is a precious gemstone that is sought-after for its rich color; it is also a strategic mineral resource due to its primary constituent, beryllium. Beryl is a hexagonal system crystal with the ideal chemical formula of Be3Al2(SiO3)6 [1,2,3]. It is a cyclosilicate mineral with a crystal structure in which six-membered rings of Si-O tetrahedra are arranged in planes perpendicular to the c-axis. The six-membered rings are then connected by Al3+ in octahedral coordination and Be2+ in tetrahedral coordination and stacked to form broad channels parallel to the c-axis [1,4,5,6]. Pure beryl is colorless. However, when the crystal lattice contains trace chromogenic elements (Cr, Fe, V, Ti, and Mn), a spectrum of colors emerges, including green (emerald), greenish-blue or blue (aquamarine), pink (morganite), yellow (heliodor or golden beryl), colorless (goshenite), and red [6,7,8].
Recently, a vanadium-bearing beryl of Nigerian origin with a distinctive green (blue-green) color has emerged within the international jewelry market. Initially misidentified as African Paraiba blue-green tourmaline due to its similar color and luster, it was subsequently determined to be vanadium-bearing beryl. Vanadium has been identified in various gemstones, including vanadium grossular (tsavorite) [9,10] and vanadium-bearing chrysoberyl [11], among others. The blue-green paraiba tourmaline, some of the bright green vanadium tsavorites, vanadium-containing chrysoberyl, and green or blue-green vanadium-containing beryl frequently exhibit an “electro-optical” green/blue-green color. “Electro-optical” is a term used in the gem market to describe gemstones that are not particularly intense but exhibit a very bright green/blue-green color. However, it should be noted that such gemstones do not have an “electroluminescent” effect. The “electro-optical” color or “electro-optical” effect is not really a professional name for the color. The “electro-optical” effect is simply the impression given by a color that is too bright. This phenomenon positively impacts the overall color of the stone, contributing to its popularity in the jewelry market. In previous reports, vanadium has frequently been identified in the company of chromium in beryl, resulting in a green hue, i.e., the formation of emeralds. The emeralds from Colombia, Afghanistan, and Malipo (Yunnan, China) are characterized by significantly higher concentrations of vanadium and chromium [12,13]. However, the coloration of the Nigerian vanadium-bearing beryl is distinctly different from that of emeralds, and there are some differences from other normal green beryls. The scarcity and high cost of vanadium-bearing beryl has been an obstacle to its study. The lack of research on vanadium-bearing beryl (distinct from emerald) has hindered understanding its coloration mechanism.
In this study, a series of non-destructive analyses, including standard gemological tests, XRF fluorescence spectroscopy, infrared, Raman, and ultraviolet spectroscopy, were used to reveal the gemological characteristics and chromogenic mechanism of this particular vanadium-bearing beryl. These analyses provide new information for the identification of the gemstone’s origin.

2. Materials and Methods

2.1. Materials

The samples in this study were obtained from a reliable French jeweler. This French jeweler acquires raw materials in Nigeria and subsequently cuts and polishes them into faceted stones for sale.
Three Nigerian vanadium-bearing beryls (designated as N01 to N03) were selected as study samples based on hue. All samples are well-cut and polished faceted gemstones that exhibited a distinctive “electro-optical” green hue with slight variations in saturation.

2.2. Methods

A comprehensive examination of the samples was conducted using standard gemological methods to ascertain their optical properties. These properties included microscopic features, refractive index (RI), double refractive index (DR), dichroism, reactions under UV fluorescence, and Chelsea color filter (CCF). Microscopic features were observed with a gemological microscope. Dichroism was measured with a calcite dichroscope. The observation of UV fluorescence was facilitated through the utilization of a UV lamp that operated under the wavelengths of 365 nm (long wave) and 254 nm (short wave), respectively. The three instruments were manufactured by Baoguang Technology (Nanjing, China). The RI and the DR were measured using a Gem-A refractometer, which was manufactured by the Gemological Association of Great Britain (London, United Kingdom). The samples’ specific gravity (SG) was determined by the hydrostatic weighing method, and the average value of three measurements was taken.
X-ray fluorescence (XRF) analysis was conducted under vacuum conditions using a Shimadzu EDX7000 (Kyoto, Japan) X-ray fluorescence spectroscopy apparatus and employing the Fundamental Parameter (FP) method to obtain compositional semi-quantitative results. The analyses were performed at 15 kV and 1000 μA for Na-Sc, and at 50 kV and 240 μA for Al-U. The FP method for quantitative analysis is a calculation that generates a “universal scale” equivalent to a standard, which is not dependent on the standard sample. The method is based on the comparison of the intensity of the X-ray fluorescence spectrum measured by the equipment with the spectrum calculated using the relevant formula. The instrument is accompanied by data analysis software (PCEDX Pro Analyzer, version 2.03) that automates this process. The detection limit of a semi-quantitative instrument is subject to variation for each element under examination. During the experimental phase, the detected spectra of each sample are thoroughly evaluated, and the elemental content values which have clearly identifiable spectral peaks are retained. While these values may be of a minor magnitude, the results obtained are of a comparatively reliable nature.
The UV-visible-near infrared (UV-Vis-NIR) spectra were obtained using a GEM-3000 UV-Vis spectrophotometer using an in situ reflection method; this spectrophotometer is made by Biaoqi Detection Technology Co., Ltd. (Guangzhou, China). The instrument was equipped with an integrating sphere that measures the reflectance of the sample and converts it to transmittance or absorbance to obtain a spectrum. The recorded spectra were analyzed over the range of 220–1000 nm at intervals of 0.5 nm, with an integration time of 220 s.
The infrared (IR) spectra of the samples were measured using a Bruker (Karlsruhe, Germany) TENSOR 27 Fourier Transform Infrared Spectrometer with a resolution of 1 cm−1. The instrument was equipped with a diffuse reflectance accessory (DRIFT), which can be used for in-situ reflectance testing of gemstone samples. The IR spectra were obtained through 32 scans in reflectance mode within the 400–2000 cm−1 range and in transmission mode within the 8000–2000 cm−1 range.
The Raman spectra were measured using an HR-Evolution HORIBA laser Raman spectrometer (manufacturer: HORIBA Scientific, Longjumeau, France) with a 532 nm laser source, 3 s integration time, and two scans at an acquisition range of 100–4000 cm−1.
Nonpolarized spectra were recorded for all samples. All analyses were performed at the Gemology Experimental Teaching Center of China University of Geosciences (Beijing, China).

3. Results

3.1. Standard Gemological Methods

All samples exhibited a glassy luster with good transparency. The RI values of the samples were 1.565–1.573. The DR values were 0.006–0.008, and the SG values were 2.66–2.69. The RI and SG values were within the range known for natural beryl, but they were at the lower end of the range. The samples were inert at both long (365 nm) and short (254 nm) UV wavelengths. They had a moderate light yellowish-green to blueish-green dichroism and appeared green (did not turn red) under the Chelsea color filter. Photographs and specific gemological information about the samples are presented in Table 1.
Despite the sample’s good clarity, some characteristic inclusions are visible under a high-magnification microscope. Hexagonal growth lines, which are similar to a typical feature of Nigerian emeralds [13], are present in the samples (Figure 1a). Veil-like fluid inclusions are prevalent in the sample (Figure 1b), similar to those found in emeralds of most origins. The presence of parallel bands is also occasionally observed (Figure 1c). Gas–liquid two-phase inclusions with an oriented arrangement in the form of tubes, long or short columns, similar to the parallel tubes with a rain-like appearance (rain-like inclusions) seen in natural aquamarines, are seen in sizes ranging from 30 to 300 µm (Figure 1d,e). Additionally, irregular, jagged, gas–liquid two-phase inclusions measuring 3–500 µm are frequently observed between the “rain-like” inclusions. On occasion, these inclusions exhibit a gas–liquid–solid three-phase composition (Figure 1f).

3.2. X-Ray Fluorescence Spectroscopy

As shown in Table 2, the main oxide contents of the samples were analyzed by X-ray fluorescence spectroscopy. The samples contained 0.63–0.98 wt.% (average 0.80 wt.%) total iron (TFeO), 0.05–0.10 wt.% (average 0.08 wt.%) V2O3, and 0.01–0.04 wt.% (average 0.02 wt.%) Cr2O3. Although XRF is semi-quantitative, the presence of V and Cr spectral peaks is evident, and the content of the same batch of tests can be utilized for concentration comparison. The TFeO content was significantly higher than the V and Cr content. Alkaline metal ions, such as Na+, exhibited weak peaks and low concentrations in the XRF spectrum. No pronounced magnesium peaks were detected, indicating that its concentration was extremely low.

3.3. UV-Vis-NIR Spectra

Figure 2 illustrates the UV-Vis-NIR spectra of all samples measured at 25 °C, the room temperature. The main absorption peaks appear as a sharp band at 371 nm, a broad band at 427, a broad band centered at 610, an extensive band centered at 810, and an absorption peak at 955 nm. The narrow absorption band that is observed at 955 nm is attributed to the stretching vibration of H2O [14]. The d-d transition of Fe3+ produces absorption peaks at 371 nm and 425 nm, with 425 nm being weaker than 371 nm [14,15,16,17,18]. There are two broad bands in the visible region near 427 nm and 610 nm, which are related to the absorption of Cr3+ and/or V3+ [12,15,19]. The positions of the leading absorption bands of Cr3+ and V3+ are approximately the same, making it difficult to distinguish the absorption of the superimposed bands. The absorption peak near 427 nm is broader and more substantial than the peak at 370 nm. This difference may be due to the superposition of Fe3+ and Cr3+ or V3+. A contribution from charge transfer between Fe2+ and Fe3+ may also be included in the 610 nm band [20,21], superimposed on the absorption bands of V3+ and Cr3+. The ultra-broad absorption band centered at 810 nm in the near infrared region is attributed to six-coordinated Fe2+ in the Al site and/or Fe2+ in the structure channels of beryl [8,15,16,18].

3.4. IR Spectral Characteristics

3.4.1. IR Spectra Measured by the Reflection Method

The structural vibrational infrared spectra of beryl are concentrated in the 400–1300 cm−1, i.e., the fingerprint region, which includes the bending vibration of the six-membered ring, Si-O, Al-O, and Be-O, and the stretching vibration [15,22,23,24]. The IR spectra of samples measured by the reflection method are shown in Figure 3. The analysis of the samples indicated the presence of distinct and prominent absorption bands, exhibiting a high degree of similarity in intensity and characteristics. As demonstrated by the IR bands of N01, the distinctive peaks are the intense peak at 1273 cm−1 and 1242 cm−1, the medium intensity peaks at 1074 and 1020 cm−1, the intense peak at 964 cm−1, the medium intensity peak at 810 cm−1, the intense peaks at 745 and 687 cm−1, the weak peak at 650 cm−1, the intense peaks at 596 and 530 cm−1, the medium intensity peaks at 492 cm−1, and the intense peak at 447 and 430 cm−1. The peak values and their respective attributions are presented in Table 3.

3.4.2. IR Spectra Measured by the Transmission Method

The IR spectra were obtained by the transmission method in the range of 2000–8000 cm−1. The absorption bands in the range of 2000–3500 cm−1 are mainly related to the chemical bonding vibrations of the chemical groups in the beryl structure channel. The absorption bands in the 3500–8000 cm−1 range are mainly related to the stretching vibrations of water in the beryl structure.
The samples’ IR spectra peak characteristics and peak assignments are shown in Figure 4 and Table 3, respectively.
Nigerian vanadium-bearing beryls exhibit a series of infrared spectral characteristics that are typical of beryl, including the vibrational band of the hydrated ion M-OH at 3111 cm−1, the weak vibrational band of Na-H at 3160 cm−1, and a distinct 3233 cm−1 absorption band (associated with [Fe2(OH)4]2+ [15]. In addition, several IR spectral features similar to those of most beryls are present, including absorption bands at 2360 and 2324 cm−1 related to the asymmetric stretching vibration of CO2 in the structural channels, and weak absorption bands at 2736, 2445 cm−1 related to Cl− [7,26,32,33]. A weak absorption peak at 2687 cm−1 and an absorption band at 2640 cm−1 are evident, corresponding to the HDO molecule and the D2O molecule, respectively [27]. Furthermore, a weak peak at 2816 cm−1 is observed, though the assignment of the peak remains uncertain.
Due to the substantial thickness of the gem sample, the absorption peaks associated with water in the range of 4000 to 3500 cm−1 were overly saturated, hindering the identification of their positions. Nevertheless, the absorption band exhibited by the other two ranges frequently served as a reliable distinguishing characteristic when attempting to identify H2O types within the structure.
All samples exhibited a sharp absorption peak at 5271 cm−1, accompanied by a flat band at 5448 and 5105 cm−1 (Figure 4). A dominant absorption band for type I H2O is observed at 7142 cm−1 (attributed to the asymmetric stretching vibration (ν3)), accompanied by a weaker absorption peak at 6816 cm−1 [3,7,27,31,33] (Figure 4). The 7070 cm−1 band, which is characteristic of type II water, is barely visible. In addition, weak peaks at 4881, 4804, 4648, 4546, and 4197 cm−1 are observed, although the assignment of these peaks remains uncertain.

3.5. Raman Spectroscopy

In the Raman spectral region below 1500 cm−1, crystal vibrations are observed, including stretching and bending vibrations of Si-O, Al-O, and Be-O [8,34]. A comparison of the spectra of the samples below 1500 cm−1 reveals a high degree of uniformity. As illustrated in Figure 5a and Table 4, the primary Raman bands are situated at 321, 394, 684, 1012, and 1067 cm−1. The weak band at 1242 cm−1 is associated with CO2 vibrations within the structural channel [3,7,24].
Furthermore, Raman spectroscopy can reveal the presence of symmetric stretching vibrational bands belonging to two types of H2O molecules: type I H2O, characterized by a peak at 3606 cm−1, and type II H2O, characterized by a peak at 3598 cm−1 [24,38]. In the Raman spectra of Nigerian vanadium-bearing beryls, the bands at 3606 cm−1 exhibit high intensity and clarity, while the bands at 3598 cm−1 are barely visible (Figure 5b). This observation indicates that the presence of type I H2O is predominant over type II H2O within the sample.
Raman spectroscopy is a reliable technique for the identification of inclusions in gemstones. The analysis of Raman spectra of gas–liquid two-phase inclusions reveals the presence of two peaks of CO2 Raman Fermi at 1387 cm−1 and 1284 cm−1 in the gaseous phase, and weak 1242 (1243) cm−1 and 1282 (1283) cm−1 peaks and broad peaks at 2900–3700 cm−1 produced by telescopic vibrations in the liquid phase of the aqueous solution containing CO2 [39]. The Raman spectra of these inclusions are displayed in Figure 6.

4. Discussion

4.1. Compositional Characterization of Vanadium-Bearing Beryl from Nigeria

The data from the X-ray fluorescence (XRF) spectroscopy analysis revealed that the samples contained 0.63–0.98 wt.% (average 0.80 wt.%) of total iron (TFeO), 0.05 to 0.10 wt.% (average 0.08 wt.%) of V2O3, and 0.01 to 0.04 wt.% (average 0.02 wt.%) of Cr2O3. The content of alkali metals and other impurity elements is minimal. The predominant chromogenic element is Fe, followed by V, which exceeds Cr. The low alkali and magnesium content of the Nigerian vanadium-bearing beryl is analogous to the compositional characteristics of the Nigerian emerald [13], which can serve as an indicator of origin.
The mineralization conditions significantly influence the process of beryl crystallization, often inducing the occurrence of isomorphous substitution within the lattice. The crystal structure of beryl features Be and Si sites, which are both surrounded by four O atoms in tetrahedral coordination, and Al sites, which are surrounded by six O atoms in an octahedral coordination (Figure 7a) [5,6]. The polymerization of SiO4 tetrahedra gives rise to six-membered rings that are parallel to (0001), which subsequently stack to form broad channels that are parallel to the c-axis (Figure 7b) [5,6]. In the octahedral site, A13+ is often replaced by trace cations, including Cr3+, Fe3+, V3+, Fe2+, Mg2+, Mn2+, and others [1,5,7]. The Al of the octahedral sites in the lattice of samples are primarily replaced by Fe2+, V3+, and Cr3+ [7], resulting in pronounced UV-Vis-NIR absorption spectra of Fe2+ and associated absorption bands of V and Cr.
The structural channels of beryl can absorb non-framework ions and molecules [7]. The most prevalent channel constituents include H2O molecules, alkali ions (such as Na+, Li+, K+, Rb+, and Cs+), and CO2 molecules (Figure 7b) [7]. Infrared and Raman spectroscopy tests have demonstrated the presence of CO2, HDO and D2O molecules in the samples examined. The vibration peak of CO2 in the infrared absorption spectrum of beryl is found to be close to that of free CO2 molecules, thereby reflecting its weak interaction with the structural channels [24].
The IR spectrum shows an extremely weak Na-H vibrational band at 3160 cm−1, indicating the presence of trace amounts of Na+ in the structural channels of the sample. This is consistent with the very low alkali metal content of the X-ray fluorescence spectroscopy data. The relationship between the content of alkali metals and the amount of substitution has been demonstrated in previous studies [1,3,7]. The low-valent ions in the octahedral and tetrahedral sites of the beryl crystal structure replace the Al and Be in the structure, resulting in a deficiency of positive charge [1,7,21,40]. Alkali ions, such as Na+, Li+, K+, Rb+, and Cs+, compensate for the insufficient charge by entering the beryl’s channel [3,15,24,41].
Most studies have determined that the entry of Na+ into beryl channels is accompanied by the coordination of H2O molecules. While beryl is traditionally considered an anhydrous mineral, it is notable that beryl structural channels frequently contain water molecules. The orientation of H-H bonds within the water molecules in these channels relative to the c-axis of beryl can be classified as type I, with H-H bonds parallel to the c-axis, or type II, with H-H bonds perpendicular to the c-axis [7,15,25] (Figure 7). The alkali cation, being positively charged, engages in interactions with the negative charges of the O atoms in the H2O molecule, thereby causing a change in its orientation to the type II H2O configuration [3,7,27,33]. Consequently, the presence and amount of this type of H2O are contingent on the presence and amount of alkali ions in the channel [7,25,42,43]. Given the established correlation between alkali content and substitution levels in beryl and the direct relationship between type II H2O and alkali ions, the amount of type II H2O is also influenced indirectly by the amount of substitution. Vanadium-bearing beryl, which is characterized by its very low alkali content and low substitution rate, exhibits predominantly Type I H2O.

4.2. Gemological Characteristics

The two RI values for various natural beryls generally range from 1.565 to 1.599 and 1.569 to 1.610 [4]. The RI values of the Nigerian vanadium-bearing beryl (1.565–1.573) and its SG values (2.66–2.69) are at the lower end of the range known for natural beryl, which is consistent with the characteristics of Nigerian emeralds [13,44,45]. Previous studies suggest that the refractive index and specific gravity values of beryl increase in conjunction with the degree of AI substitution within the beryl lattice and the concentrations of alkali elements (Li, Na, K, Cs) [4,46]. The low Mg and alkali contents, as indicated by XRF data, and the predominant presence of type I water in the sample channels, as indicated by IR and Raman spectroscopic tests, are indicative of the minimal AI substitution in the Nigerian vanadium-bearing beryl lattice structure. Consequently, its refractive index and specific gravity values are low. The UV fluorescence inertness and the absence of reddening under CCF can be attributed to the low chromium and rich iron content in the sample.
The Nigerian vanadium-bearing beryl is characterized by a high density of fluid inclusions, indicative of a mineralizing environment with abundant fluid presence. These inclusions exhibit a combination of characteristics found in emeralds and aquamarines. Specifically, these inclusions exhibit characteristics such as orientation, as seen with raindrop inclusions and cavities within the gas-liquid phase, as observed in aquamarines. A notable similarity is observed between these two-phase inclusions and those reported in emeralds from Brazil, Zambia, Russia, and Ethiopia [13,38,47]. Irregular, jagged, gas–liquid two-phase inclusions (sometimes three-phase) are also typical (Figure 1f) and are morphologically similar to fluid inclusions in Colombian, Afghan, and Davdar (Xinjiang, China) emeralds [13,47,48,49,50]. However, inclusions of this morphology tend to be three-phase in emeralds of these origins. Hexagonal growth lines and specific elongated linear inclusions are also present. Mineral inclusions were not observed in the samples examined in this study.

4.3. Unique “Electro-Optical” Green Mechanism

The primary UV-Vis-NIR absorption peaks of the samples manifest as the sharp peak at 371 nm, the broad peak at 427 nm, the broad peak centered at 610 nm, and the vast peak with a center at 810 nm. The broad absorption peaks near 427 nm and 610 nm in the visible region are related to the absorption of Cr3+ and/or V3+ [15]. The positions of the leading absorption bands of Cr3+ and V3+ are approximately similar, and the overlapping absorption is difficult to distinguish. The d-d transition of Fe3+ generates sharp peaks at 371 nm and 425 nm, with the latter being weaker than the former [14,15,16,17,18]. Nevertheless, the sample’s absorption peak at 427 nm is broad and significantly more substantial than the peak at 370 nm. This discrepancy can be attributed to the superposition of absorption bands of Fe3+ and Cr3+, or possibly V3+.
The sample N01 (0.04 wt.% Cr2O3, 0.07 wt.% V2O3) exhibited a series of weak UV-Vis-NIR absorption peaks associated with Cr3+ at 645, 662, and 684 nm [15], while sample N02 (0.02 wt.% Cr2O3, 0.06 wt.% V2O3) showed a weak peak at 684 nm. The observed peaks indicate the presence of trace amounts of Cr3+, and the amount of Cr is proportional to the intensity of the peak in question. However, beryl with only V3+ as a chromophore does lack these three peaks. No identifiable peaks are associated with Cr3+, as seen in sample N03, which contains 0.01 wt.% Cr2O3 and 0.10 wt.% V2O3. Sample N03 (V-rich) displays a comparable range of color saturation to sample N01 (total V and Cr content approaching the V content of N03), indicating that the chromophore effectiveness of V3+ in beryl is comparable to that of the Cr3+. This hypothesis has been supported by several studies of V and Cr in emeralds (e.g., Yunnan Malipo emerald) [12].
The ionic radii of V3+ (0.640 Å) and Cr3+ (0.615 Å) are comparable to the ionic radius of Al3+ (0.535 Å) in the six-fold-coordinated octahedral sites [51], which suggests a high degree of compatibility between Al3+ and either V3+ or Cr3+ in the AlO6 octahedral sites. Furthermore, Cr3+ in beryl has been observed to form significant UV-Vis-NIR spectral absorption bands at approximately 430 nm and 600 nm (o-ray) and 420 nm and 630 nm (e-ray) [12,15]. V3+ in the AlO6 octahedra of beryl forms the two most significant UV-Vis-NIR spectral absorption bands at 432 nm and 611 nm (o-ray) and 425 nm and 644 nm (e-ray), as well as an absorption shoulder at 395 nm (o-ray and e-ray) [12,19,29]. The anisotropic absorption of V3+ at 425–432 nm is pronounced, and the maximum absorption coefficient of o-rays at 425–432 nm is nearly twice that of e-rays [12]. Consequently, the absorption induced by V3+ in the visible violet-blue region exhibits stronger intensity and a greater tendency towards the blue region than Cr3+, resulting in a more yellowish-green coloration of beryl [12]. Nigerian vanadium-bearing beryl, with a notably elevated vanadium content compared to chromium (particularly in sample N03), shows significant absorption of V3+ ions in the visible spectrum.
It has been determined that the light blue color exhibited in beryl (aquamarine) is caused by ferric (Fe) as the predominant chromophore [6,7,8,14]. Nevertheless, the precise mechanism by which Fe engenders this coloration in these gemstones remains a subject of debate. The UV-Vis-NIR spectra of all the samples exhibit Fe absorption bands (Figure 2). The absorption bands at 371 nm (attributed to the 6A1 → 4E(4D) transition of Fe3+) and 425 nm (resulting from the A1g → 4Eg +4A1g transition of Fe3+, which superimposes on the spectrum of V, forming a 427 nm broad band in the samples) are associated with the spin-forbidden d-d transition of Fe3+ in the octahedral coordination [14,15,16,17,18]. The Fe3+-induced absorption band exerts minimal effect on the beryl coloration [43]. Previous studies have shown that two main factors may be responsible for the blue coloration of beryl. First, the UV-Vis-NIR broad absorption band centered at 610 nm is associated with charge transfer between Fe2+-Fe3+ pairs [20,21]. The Nigerian vanadium-bearing beryl displays an absorption band centered at 610 nm that may be induced by the charge transfer between Fe2+ and Fe3+, superimposed on the absorption band near 610 nm produced by the V3+ and/or Cr3+. Secondly, the ultra-broad absorption band, which is centered at 810 nm in the near infrared region, is associated with six-coordinated Fe2+ in the Al site (induced by spin-allowed transitions 5A1(5T2) → 5E(5E)) and/or Fe2+ in the channel cavities of beryl [8,15,16,18,21,43]. Despite not being a typical chromophore, Fe2+ yields a broad absorption band centered at 810 nm within the near-infrared region, which can extend to shorter wavelengths reaching the visible red region [8]. This may result in a light blue color for beryl. However, the UV-Vis-NIR spectrum of vanadium beryl suggests that the contribution of Fe2+ to its color may be negligible.
Nigerian vanadium-bearing beryl contains iron comparable to aquamarine and trace elements of V and Cr (lower than the average emerald). When the V content exceeds the Cr content, the color effect induced is 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. In contrast to light green emeralds and blue aquamarines, “electro-optic” green (blue-green) beryls appear extra bright, but their body color is not yellow-green.

5. Conclusions

Nigerian vanadium-bearing beryl is distinguished by its low alkali and magnesium composition, with iron (Fe) serving as the predominant chromogenic element, followed by vanadium (V), which exceeds chromium (Cr) levels. The content of impurity elements and degree of Al substitution is very low, resulting in a lower refractive index and specific gravity value compared to most beryl. Spectroscopic analyses indicate that the structural channel is dominated by Type I H2O, with CO2, HDO and D2O molecules also present. The inclusions observed in vanadium-bearing beryl are similar 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 inclusions and significant 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.

Author Contributions

Data curation, Y.H., X.S. and Y.M.; Formal analysis, Y.H., X.S. and Y.M.; Investigation, X.S., Y.M. and Y.Y.; Methodology, Y.H. and X.S.; Project administration, Y.Z.; Resources, Y.Z.; Software, Y.H. and Y.Y.; Supervision, Y.Z.; Validation, Y.H.; Writing—original draft, Y.H. and Y.Z.; Writing—review & editing, Y.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Innovation and Entrepreneurship Training Program for College Students of China University of Geosciences (Beijing) (A448).

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Acknowledgments

The authors would like to thank Ye Yuan for his technical support in the laboratory (School of Gemology, China University of Geosciences, Beijing). The authors appreciate the professional review work, constructive comments, and valuable suggestions of the three anonymous reviewers.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Minerals 15 00557 g001
Figure 1. Microscopic images of vanadium-bearing beryl inclusions: (a) hexagonal growth lines, (b) veil-like fluid inclusions, (c) parallel bands, (d) irregular and jagged gas–liquid two-phase inclusions frequently observed between "rain-like" inclusions, (e) gas–liquid two-phase inclusions in the form of tubes, and (f) gas–liquid–solid three-phase inclusions. L—liquid phase; V—gas phase; S—solid phase.
Figure 1. Microscopic images of vanadium-bearing beryl inclusions: (a) hexagonal growth lines, (b) veil-like fluid inclusions, (c) parallel bands, (d) irregular and jagged gas–liquid two-phase inclusions frequently observed between "rain-like" inclusions, (e) gas–liquid two-phase inclusions in the form of tubes, and (f) gas–liquid–solid three-phase inclusions. L—liquid phase; V—gas phase; S—solid phase.
Minerals 15 00557 g001
Minerals 15 00557 g002
Figure 2. UV-Vis-NIR spectra of the samples collected in the 250–1000 nm range.
Figure 2. UV-Vis-NIR spectra of the samples collected in the 250–1000 nm range.
Minerals 15 00557 g002
Minerals 15 00557 g003
Figure 3. IR spectra of the samples in the 400–1500 cm−1 range measured by the reflection method.
Figure 3. IR spectra of the samples in the 400–1500 cm−1 range measured by the reflection method.
Minerals 15 00557 g003
Minerals 15 00557 g004
Figure 4. IR spectra of the samples in the 2000–7500 cm−1 range measured by the transmission method.
Figure 4. IR spectra of the samples in the 2000–7500 cm−1 range measured by the transmission method.
Minerals 15 00557 g004
Minerals 15 00557 g005
Figure 5. (a) Raman spectra of samples in the 100–1500 cm−1 range. (b) The characteristic band at 3606 cm−1 of Type I H2O.
Figure 5. (a) Raman spectra of samples in the 100–1500 cm−1 range. (b) The characteristic band at 3606 cm−1 of Type I H2O.
Minerals 15 00557 g005
Minerals 15 00557 g006
Figure 6. Raman spectra of two-phase gas–liquid inclusions in samples: (a,b) irregular, jagged inclusions; and (c,d) inclusion in the form of tube.
Figure 6. Raman spectra of two-phase gas–liquid inclusions in samples: (a,b) irregular, jagged inclusions; and (c,d) inclusion in the form of tube.
Minerals 15 00557 g006
Minerals 15 00557 g007
Figure 7. (a) The beryl structure, looking down the c axis of the crystal. Structural channels can be filled with various different ions, such as Na or Cs, Rb or K (alkali ions), or molecules such as CO2 or type I H2O and type II H2O (H2O in different orientations). (b) The schematic diagram of the beryl structure channels shows the structural oxygen atoms surrounding the channel constituents.
Figure 7. (a) The beryl structure, looking down the c axis of the crystal. Structural channels can be filled with various different ions, such as Na or Cs, Rb or K (alkali ions), or molecules such as CO2 or type I H2O and type II H2O (H2O in different orientations). (b) The schematic diagram of the beryl structure channels shows the structural oxygen atoms surrounding the channel constituents.
Minerals 15 00557 g007
Table 1. Photographs and gemological properties of Nigerian vanadium-bearing beryls.
Table 1. Photographs and gemological properties of Nigerian vanadium-bearing beryls.
PropertyN01N02N03
ImageMinerals 15 00557 i001Minerals 15 00557 i002Minerals 15 00557 i003
Color“electro-optical” green“electro-optical” green“electro-optical” blue-green
Size (mm)5.88 × 4.06 × 3.334.57 × 4.55 × 4.006.15 × 4.28 × 3.46
Weight (ct)0.470.620.74
Refractive index (RI)1.565–1.5731.567–1.5731.567–1.573
Double refractive index (DR)0.0080.0060.006
DichroismWeak, yellowish green/blueish-greenWeak, yellowish green/blueish-greenWeak, yellowish green/blueish-green
Specific gravity (SG)2.672.692.66
UV fluorescence (LW)InertInertInert
UV fluorescence (SW)InertInertInert
Chelsea color filter (CCF)greengreengreen
Microscopic featuresTwo-phase gas–liquid parallel tubes with rain-like appearance (rain-like inclusions)Two-phase gas–liquid parallel tubes with rain-like appearance (rain-like inclusions), irregular, jagged multiphase inclusions, hexagonal growth lines, and parallel bands.Two-phase gas–liquid parallel tubes with rain-like appearance (rain-like inclusions), veil-like fluid inclusions.
LW: long-wave (365 nm); SW: Short-wave (254 nm).
Table 2. The major oxide contents of the samples.
Table 2. The major oxide contents of the samples.
SampleN01N02N03
wt.%Std. Dev.wt.%Std. Dev.wt.%Std. Dev.
SiO276.480.91776.480.9276.210.944
Al2O322.360.81422.510.81922.660.85
TFeO0.790.0080.630.0070.980.011
V2O30.070.0010.060.0010.10.002
Cr2O30.040.0010.020.0010.010.001
BaO0.150.012ndl. ndl.
CaO0.060.0010.040.0010.030.001
CuO0.010.0010.020.001ndl.
ZnOndl. 0.060.001ndl.
K2Ondl. 0.17 ndl.
Total99.96 99.98 99.95
V2O3av0.08
Cr2O3av0.02
TFeOav0.8
Note: TFeO = total Fe contents, av = average value. Std. dev. = standard deviation.
Table 3. Infrared spectral band characteristics of Nigerian vanadium-bearing beryls in the region 400–8000 cm−1 and their assignments.
Table 3. Infrared spectral band characteristics of Nigerian vanadium-bearing beryls in the region 400–8000 cm−1 and their assignments.
Band Assignments [1,5,15,22,23,25,26,27,28,29,30,31]Nigerian Vanadium-Bearing Beryls
N01
(cm−1)		N02
(cm−1)		N03
(cm−1)
ν3 H2O typeI714271407140
ν H2O typeII70707055n.d.
ν H2O typeI681668166814
H2O typeI544854485444
ν2+ν3 H2O typeI/II527152705270
ν H2O typeI510551015109
488148774875
480448044804
454645464546
419741974197
[Fe2(OH)4]2+323332333233
Na-H316031603159
M-OH311131113111
281628162816
Cl−273527362735
HDO268726872687
D2O264026402642
Cl−244524412445
νCO2236023582362
νCO2232423202324
ν3(Si-O-Si)127312751275
ν3(Si-O-Si)124212441252
ν3(Si-O-Si)107410681074
ν3(O-Si-O)102010181020
ν1(O-Si-O)964974965
ν1(Si-O-Si)810808810
ν1(Si-O-Si)745752746
ν1(Si-O-Si)687689687
ν1(Be-O)650650650
ν2(Si-O), ν(M-O)596598596
ν2(Si-O), ν(M-O)530532532
ν2(Si-O), ν(M-O)492494494
ν2(Si-O), ν(M-O)447445453
ν2(Si-O), ν(M-O)430435430
Abbreviations: n.d., not detected; M, metal ion; ν1, the symmetric stretching vibration; ν2, the bending vibration; ν3, the asymmetric stretching vibration.
Table 4. Raman peaks of Nigerian vanadium-bearing beryls and their assignments.
Table 4. Raman peaks of Nigerian vanadium-bearing beryls and their assignments.
Band Assignments
[3,23,24,25,34,35,36,37]		Nigerian Vanadium-Bearing Beryl
N01
(cm−1)		N02
(cm−1)		N03
(cm−1)
144146144
253253253
E1g+E2g overlapn.d.292294
A1g+E2g overlap321323321
A1g+E2g overlap394396396
E2g421423421
E1g+E2g overlap446449446
ν(Al-O), E1g526530528
E2g582582584
A1g622622n.d.
ν(Be-O), A1g+E2g684684684
ν(Al-O), E1g+E2g overlap769772771
ν(Si-O), E1g+E2g916914916
ν(Si-O), E1g+E2g101210121010
ν(Si-O), A1g106710681067
ν1 CO2, E2g124212421242
ν1 H2O type I360636063606
Abbreviations: n.d., not detected; M, metal ion; ν, the vibration; ν1, the symmetric stretching vibration.
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Hong, Y.; Zhang, Y.; Shao, X.; Mu, Y.; Yu, Y. Gemological Characteristics and Coloration Mechanism of Vanadium-Bearing Beryl from Nigeria. Minerals 2025, 15, 557. https://doi.org/10.3390/min15060557

AMA Style

Hong Y, Zhang Y, Shao X, Mu Y, Yu Y. Gemological Characteristics and Coloration Mechanism of Vanadium-Bearing Beryl from Nigeria. Minerals. 2025; 15(6):557. https://doi.org/10.3390/min15060557

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Hong, Yunlong, Yu Zhang, Xinyi Shao, Yanyi Mu, and Yuemiao Yu. 2025. "Gemological Characteristics and Coloration Mechanism of Vanadium-Bearing Beryl from Nigeria" Minerals 15, no. 6: 557. https://doi.org/10.3390/min15060557

APA Style

Hong, Y., Zhang, Y., Shao, X., Mu, Y., & Yu, Y. (2025). Gemological Characteristics and Coloration Mechanism of Vanadium-Bearing Beryl from Nigeria. Minerals, 15(6), 557. https://doi.org/10.3390/min15060557

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