Sourcing

A New Source in Tanzania Is Producing Cobalt-Blue Spinel

SourcingMay 17, 2022

A New Source in Tanzania Is Producing Cobalt-Blue Spinel

The stones come from a deposit close to Mahenge and have been on the market for several months.

Examples of the cobalt-blue spinel from Tanzania (Image courtesy of Mahenge Gems)
New York—Tanzania’s Mahenge region is known in the gem world for its beautiful red and pink spinel, but the area has begun producing another color recently—cobalt blue.

The newly discovered deposit is about 12.5 miles southeast of Mahenge.

The spinel started appearing on the market late last year, according to wholesaler Mahenge Gems, which was established in early 2020 and has direct access to miners in the area.

The material began arriving in the Mahenge Gems office around late October/early November, Director Wez Barber said.

At the beginning, they thought it was standard blue spinel because of the large size of the crystals.

The leading source of cobalt-blue spinel is Vietnam’s Luc Yen District, known for its vibrant blue material, though those stones tend to be small. 

This Tanzanian material, Barber said, is the “best of both worlds”—good quality, clean, saturated stones that are more than 1.5 carats in size, including some as big as 40 carats. 

The colors so far have ranged from a denim blue to a vibrant electric blue, Barber noted. 

“We bought as much of it as we could lay our hands on, and still are, because it was such nice material,” he said. 

The company sent stones to a few different labs to have it confirmed as cobalt-blue spinel. 

 Related stories will be right here … 

Swiss gem lab SSEF analyzed a selection of the material, confirming cobalt as the main coloring element, in combination with iron.

This makes the Mahenge material similar to cobalt spinel from sources in Sri Lanka, Pakistan, and Vietnam, the lab said.

The Tanzanian material contains characteristic inclusions, SSEF noted, like oriented geometric lamellae with interference colors together with lines of fine particles. It said similar inclusions have been found in red spinel from Morogoro in eastern Tanzania.

Through Raman spectroscopy, SSEF also found colorless apatite and clusters of small zircon inclusions.

Because the deposit was discovered fairly recently, it remains to be seen how much material it will produce.

Barber said that in the past six weeks to two months, very little material has been coming through their office, but he also noted there had been little mining activity due to the rainy season and Ramadan.

With Ramadan ending in early May and the rainy season coming to a close, they should soon be able to ascertain how much more material the deposit might produce.

Those interested in the cobalt-blue spinel can contact Mahenge Gems via its website or follow the company on Instagram

Mahenge Gems will also have the Tanzanian cobalt-blue spinel on display during the upcoming Las Vegas Market Week. Visit them in the Gems pavilion at JCK Las Vegas at Booth 33105. 
Brecken Branstratoris the senior editor, gemstones at National Jeweler, covering sourcing, pricing and other developments in the colored stone sector.

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