For several months already a supply of blue cobalt-bearing spinel has been appearing in the gem trade, including some stones of remarkable size –up to 40 carats – and quality. According to information from reliable sources, the source of the new find is a deposit located about 20 kilometres south-east of Mahenge, Tanzania, which for more than 20 years has been known for its red to pinkish red spinel of exceptional quality.
Figure 1: Cobalt-blue spinel from a newly reported source southeast of Mahenge in Tanzania, together with a vivid pinkish red spinel from a deposit in Epanko, which is close by to Mahenge, Tanzania. Photo: SSEF
Detailed analyses of a selection of blue spinels from the newly reported source, conducted at the Swiss Gemmological Institute SSEF, has revealed that they contain cobalt as a main colouring element (chromophore), in combination with iron. This is similar to cobalt-spinel materials from sources in Sri Lanka, Pakistan and Vietnam.
Interestingly, reported SSEF, cobalt-blue spinel from the source in Tanzania contains quite characteristic inclusion features, such as oriented geometric lamellae with interference colours (Figure 2), together with lines of fine particles. Similar inclusion features have been described in red Tanzanian spinel from Morogoro. Additionally, colourless apatite and clusters of small zircon inclusions were identified by Raman spectroscopy.
Figure 2: Oriented lamellae with interference colours together with lines of fine particles are characteristic features of the newly reported cobalt-blue spinels from Tanzania. Photo: M.S. Krzemnicki, SSEF
Gem-quality cobalt-blue spinels are highly sought-after and valued, and consequently have been studied extensively by SSEF. Its laboratory has offered origin determination for cobalt-blue spinel for many years.
“Cobalt-blue spinel from this new source in Tanzania is a welcome and attractive addition to the trade”, said Dr. Michael Krzemnicki, Director at the Swiss Gemmological Institute SSEF.
“It is also fascinating material for us gemmologists to study, as these spinels display specific gemmological characteristics that are helpful for origin determination.”