The most unusual and exquisite things are often found in unexpected places. This has been seen in the explosion in gem discoveries in East Africa since the latter part of the twentieth century. However, West Africa has remained relatively underexplored, despite the development of some promising and productive gem deposits, including sapphire and Paraíba tourmaline in Nigeria. West Africa’s gemological horizons expanded earlier this year with the find of a small batch of aquamarine with exceptionally deep and saturated color (figure 1). Reportedly found in Nasarawa State, the stones were recovered by small-scale artisanal miners whose workings reached only the near-surface exposure of a weathered pegmatite body. Mining was short-lived, starting likely in January and ending in May 2019, when the miners exhausted the surface deposit but lacked resources to continue with hard-rock mining once they hit bedrock. Through a local buyer in the Abuja market, author JH was able to obtain a large parcel that may account for the lion’s share of the production from this deposit to date. This parcel amounted to 763 grams, of which 200 grams would cut stones over one carat.
Figure 2. Two wafers of the new Nigerian aquamarine in unpolarized light (left), and with light polarized along the extraordinary ray (center) and along the ordinary ray (right). Photos by Aaron Palke.
Figure 3. UV-Vis-NIR absorption spectra of a polished plate of Nigerian aquamarine showing both the ordinary (o-ray) and extraordinary (e-ray) ray absorption.
Standard gemological testing showed properties consistent with aquamarine, with a uniaxial negative optic character, a refractive index of 1.582–1.590, a birefringence of 0.008, and a specific gravity of 2.71–2.73. The stones were inert under short-wave and long-wave UV light. The most exceptional feature of this new find of aquamarine—besides its attractive, saturated blue color—is its unusually strong pleochroism, going from deep, saturated blue with light polarized parallel to the extraordinary ray to an unsaturated (almost colorless) greenish blue with light polarized parallel to the ordinary ray (figure 2). The origin of this extreme pleochroism is seen in the UV-Vis spectra (figure 3), in which the main feature is a highly polarizable broad absorption band at approximately 680 nm caused by Fe2+-Fe3+ intervalence charge transfer (IVCT). This IVCT absorption is the cause of blue coloration in (non-Maxixe) aquamarine (e.g., I. Adamo et al., “Aquamarine, Maxixe-type beryl, and hydrothermal synthetic blue beryl: Analysis and identification,” Fall 2008 G&G, pp. 214–226). Also seen are an absorption band at 835 nm related to octahedral Fe2+ and narrow absorption bands at 372 and 428 nm related to octahedral Fe3+. The reason for the intense pleochroism is simply the increased intensity of the highly polarizable Fe2+-Fe3+ IVCT chromophore relative to most other aquamarine. Note that the two wafers in figure 2 were cut from the same piece of rough and were used in a pilot heat treatment experiment. The larger one on the left was not heated, while the smaller piece on the right was heated at 400°C for two hours, with no apparent change in its color or appearance. It is not clear, however, if any of the material had been heated before it reached the market; further experiments may be warranted.
Figure 4. Needle and platelet inclusions are ubiquitous in this new find of Nigerian aquamarine, as seen with transmitted light polarized along the o-ray (left) and e-ray (right). Photomicrographs by Nathan Renfro; field of view 1.43 mm.
Microscopic observations showed an abundance of highly reflective, opaque, elongate needles and platelets that appeared light brown to black in darkfield and transmitted light (figure 4). Confocal Raman spectroscopy could not isolate the inclusions, but their dark color and reflective nature suggest they might be Fe oxides, which would be consistent with the iron-rich nature of these stones.
Trace element content of the Nigerian aquamarines was measured by LA-ICP-MS (table 1). They were extremely enriched in alkali metals, especially Li, K, and Na (up to 9400 ppm). To the authors, these high concentrations of alkali metals seem unusual for gem-quality aquamarine among global deposits (including saturated material from Santa Maria in Minas Gerais, Brazil). The unique chemistry of this new material almost warrants the use of the term “alkali beryl,” even though this term is nonstandard in the mineralogical community. Also notable is the relative enrichment of Cr from 4 to 49 ppm, compared to around 1 ppm Cr or less in aquamarine from most other deposits. The Cr enrichment was also indicated by the prominent Cr fluorescence measured by photoluminescence spectroscopy. Finally, the deep blue color of these stones was explained by their relative enrichment in Fe from 11,000 to 12,600 ppm. Such a high concentration of Fe increases the probability of having adjacent Fe2+ and Fe3+ cations, thereby facilitating and enhancing the Fe2+-Fe3+ IVCT interaction. For now, this new deposit of fine Nigerian aquamarine lies dormant, but the world awaits the next important gem discovery in West Africa.
