GEMSWORLD | 41 | 2023 精辟洞见 Peridot in bespoke cuts by Mark Nuell Mark Nuell的橄榄石作品呈独特切割形态 Sean Gilbertson, CEO of Gemfields Gemfields首席执行官Sean Gilbertson Cobalt spinels fromMahenge, Tanzania 坦桑尼亚马亨盖地区的钴尖晶石 Gemfields’ Kagem emerald mine in Zambia Gemfield位于赞比亚的卡棋穆祖母绿矿 Blue wonders Spinel and peridot – two gemstones that are soaring in popularity – are also getting a significant supply boost with the discovery of new deposits of quality material. In June 2022, Mahenge Gems brought to market cobalt spinel from a new deposit approximately 12.5 miles southeast of Mahenge, Tanzania. From October 2021, miners started bringing in sizeable specimens of blue crystals suspected to be the rare cobalt spinel rather than the regular blue spinel. Swiss Gemmological Institute SSEF conducted a detailed analysis of a selection of the material, confirming cobalt as the main colouring element (chromophore), in combination with iron. SSEF found that the inclusions in these Tanzanian cobalt-blue spinels had similarities to the red spinel from Morogoro in eastern Tanzania, namely oriented geometric lamellae with interference colours together with lines of fine particles. Through Raman spectroscopy, SSEF also found colourless apatite and clusters of small zircon inclusions. “Cobalt-blue spinel from this new source in Tanzania is a welcome and attractive addition to the trade,” said SSEF Director Dr. Michael Krzemnicki. “It is also fascinating material for us gemmologists to study, as these spinels display specific gemmological characteristics that are helpful for origin determination.” Though chemically similar to the cobalt-spinel material from sources in Vietnam, the Tanzanian stones come in 1 carat to 40 carats, and their colours have ranged from a neon electric blue to a denim blue. Cobalt blue spinels from Vietnam’s Luc Yen district are usually 1 carat and below. And while they are often described as a “neon, Windex-y blue,” they tend to be lighter and more pastel than the electric blues discovered in Mahenge. Some of the newfound cobalt blue spinels are also neon and pure blues with no trace of grey. Wez Barber, director of Mahenge Gems, said the Tanzanian cobalt spinels have caught the attention of collectors, investors and jewellery designers. “We are targeting primarily Japan, Europe and the US – mature markets that have a higher concentration of gemstone enthusiasts and professionals that understand the true rarity of this material,” he shared. Mahenge Gems is moving steadily along the purchasing cycle for the new material. Demand is especially robust for investment-grade, super-rare sizes like those over 4 carats, Barber disclosed. Top-colour stones under 1 carat also move well, particularly among hobbyists and gem collectors. Production from the original pocket in Tanzania ended in April 2022. With the dwindling supply, trade prices for the natural cobalt blue spinels from Mahenge Gems now start
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