Out of the Blue | Blue Cobalt Spinel from Mahenge, Tanzania 

“These blue crystals had superb colour saturation that rivaled the Luc Yen material, but we were still shocked to receive the lab reports that stated they were indeed ‘Cobalt Spinel’,” said Barber.

Since the 2007 discovery of the 52 kg spinel rough, Tanzania’s Mahenge region has been in the limelight for hot pink and red Spinel. 15 years later, perhaps instead of the fiery hues, it’s the cool blues that will grab the world’s interest. Namely, Blue Cobalt Spinel. 

On a hunt for Spinel of all kinds, Garnet and no heat Tanzanite, Barber and Rimsan established Mahenge Gems in early 2020, just before Covid halted much of the world. By late 2021, amongst the material which came through their doors, miners were starting to bring in the most beautiful blue crystals of various carat weights, including some very sizeable specimens. From their electric hues, multi-generational gemstone supplier Rimsan did suspect that these crystals might be the extremely rare and coveted Cobalt Spinel rather than standard Blue Spinel. However, because Cobalt Spinel from leading sources such as Vietnam’s Luc Yen District tend to be 1 carat and below, Barber and Rimsan were still cautious about celebrating their new find. 

Cobalt Spinel or not, “we bought as much of it as we could lay our hands on because it was such nice material,” Barber said. 

The ‘gamble’ paid off. The company sent the stones to Swiss gem lab SSEF, GRS and ICA GemLab to have it confirmed as cobalt Spinel. 

Swiss Gemmological Institute SSEF, conducted a detailed analysis of a selection of the material, confirming cobalt as the main coloring element (chromophore), in combination with iron. While this Tanzanian material is chemically similar to the cobalt-spinel material from sources in Vietnam, the colours have ranged from a neon electric blue to a denim blue while sizes range from 1.5 carats to 40 carats.

Dr. Michael Krzemnicki, Director at the Swiss Gemmological Institute SSEF noted that, “It is also fascinating material for us gemmologists to study, as these spinels display specific gemmological characteristics that are helpful for origin determination.”

The SSEF found that the inclusions in these Tanzanian Cobalt 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. 

A question that has been repeatedly asked is, ‘how much supply do you think there is?’ As the deposit was discovered less than a year ago, the amount of material to be uncovered remains a mystery. Between March and May 2022, barely any material had been found, but it might be due to reduced mining activity over the rainy season and Ramadan.

Mahenge Gems will be at JCK Las Vegas this June at Booth 33105 so make an appointment to see these electric beauties in person! 


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