Tanzanian cobalt-blue spinels have caught the attention of collectors, investors and jewellery designers since Mahenge Gems brought them to market in June 2022.

Though chemically similar to the cobalt-spinel material from sources in Vietnam, the stones from a new deposit some 12.5 miles southeast of Mahenge, Tanzania 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, “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.” 

Mahenge Gems is moving steadily along the purchasing cycle for the new material. Demand is especially strong 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 from US$3,000, up from US$2,000 at launch, to per-carat prices that rival that of Vietnam’s Luc Yen cobalt spinels.

Completely untreated, the gems are certified by SSEF, GRS GemResearch Swisslab AG and ICA GemLab.

“With the new Mahenge source, there is a lot of potential for cobalt spinels to be used in high jewellery,” Barber said. Case in point is Singapore jewellery design house MADLY, which features three cobalt spinels in its Interlace collection and has created individual pieces for its clients.