On a Hunt for Cobalt-Blue Spinels in Mahenge 

Why blue cobalt-bearing spinels from Mahenge should be on your radar.

As far as mining locations go, Tanzania is pretty much on everyone’s radar – and Mahenge in particular, since the discovery of the 52kg rough of the finest hot pink-red spinel in 2007. The Mahenge district in Tanzania is home to all kinds of red spinels of exceptional quality and garnets, but most amazing is the discovery of a new deposit about 12.5 miles southeast of Mahenge: the rarest spinel type of them all – Cobalt Spinel in a blue that is both beautiful and fascinating. 

The colour of the Mahenge Cobalt Spinel ranges from an electric blue through to a ‘denim’ type of hue. In terms of vibrancy, it is far more vibrant than even a ‘Royal Blue’ sapphire, and while it is obviously compared, the rare cobalt-blue spinel is unique in its own right.

For years, the Cobalt-blue Spinel from the original Luc Yen deposits, in Vietnam, has wooed gem buyers and connoisseurs alike. The Luc Yen spinel is often described as ‘neon, Windex-y blue’ but it tends to be lighter and more pastel than the electric blues that have been recently discovered in Mahenge. Some of the newfound cobalt blue spinels are also neon (the best ones are) and are pure blues with no trace of grey. They are similar to how the ‘Jedi’ hot pink spinel (alluding to the Star Wars reference and the laser sword) is known for its overwhelming ‘fluorescent’ glow. 

According to the Gemological Institute of America (GIA), violet to blue spinel can be coloured by trace amounts of iron, and vibrant blues owe their saturated colour to trace amounts of cobalt. With a hardness of 8 on the Mohs scale, spinel is one of the hardest gemstones around – and often used extensively in jewellery to add pops of colour or accent the design. 

Colour, clarity, cut and carat weight are factors that determine the quality of coloured gemstones. There are two other aspects – origin, polish – that together with the 4Cs affect the gem’s beauty and value. Cuts tend to work differently on different materials. In the case of the precious spinel crystal with a cobalt hue, skilful faceting is required to present its best possible colour. 

Creating an attractive shape – one that accentuates the gem’s advantages and minimises the flaws – is a key step in the process. The rough spinel crystal dictates the final look alongside the gem-cutting process. These roughs are cut and polished in lapidaries in Tanzania, Sri Lanka and Bangkok. A cut that works well to enhance the ‘cobalt blue’ colour of the Mahenge spinel involves a brilliant crown and a step cut pavilion. The gem can be cut with an average stone depth to maintain the colour saturation. 

These cobalt blue spinels from Mahenge Gems are untreated in any manner – they are not heated, irradiated, filled or subjected to any form of treatments that are often done in the gem industry. All of the Cobalt Blue Spinel that is faceted and sold are 100 percent natural. 

The trade price for the Natural Cobalt Blue Spinels from Mahenge Gems start from USD 5,000 per carat to per carat prices which rival that of Vietnam’s Cobalt Luc Yen Spinel. These gems are certified by Gem Research Swiss Lab (SSEF), GRS GemResearch Swisslab AG, and ICA GemLab.


On Key

Related Posts