An Aquamarine Assortment

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. 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.

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.