The recent arrival of high-quality blue sapphire from a new source in Africa has caused considerable excitement in the Bangkok marketplace. These gems, reportedly from the Mambilla Plateau in Nigeria’s Taraba State, are available in large rough sizes, some as big as 100–300 ct. Like previously documented Nigerian material, these sapphires are mined from basalt-related secondary deposits. Yet the quality is significantly better than any seen to date from those sources. Initial indications suggest that this new deposit might provide a winning combination of size, high clarity, attractive color, and good crystal habit from a cutter’s perspective. Indeed, one especially fine stone is rumored to have sold for in excess of US$1 million.
As a preliminary study for a future field expedition to this location, researchers from GIA’s Thailand laboratory obtained a selection of 30 fine-quality rough sapphires from a trusted supplier in Bangkok. Representative samples were fabricated into precisely oriented wafers for the collection of high-quality optical data using FTIR and UV-Vis-NIR spectroscopy. Inclusions were identified using Raman spectroscopy, and trace-element chemical composition was analyzed using LA-ICP-MS.
A new report by the GIA laboratory in Bangkok, “Blue Sapphires from the Mambilla Plateau, Taraba State, Nigeria,” presents their initial findings. Although the inclusion suites, spectroscopic features, and LA-ICP-MS trace-element results appear similar to those of sapphires from other basalt-related deposits—including Cambodia, South Vietnam, and some Australian localities—the composition of minute “nano-inclusions” within some of the gems appears promising for origin determination purposes.
