gems, spinel, cobalt-blue spinel, Tanzania, spinel inclusions

In September 2021, a new deposit of Co-bearing blue spinel was discovered in the Lukande area, south of Mahenge in central Tanzania. We analysed 44 faceted spinels from this source, ranging from Co-dominated blue to Fe-dominated (greyish) blue. Interestingly, nearly all of these spinels showed characteristic inclusion features, consisting predominantly of oriented rhombic lamellae (inferred to be högbomite), as well as oriented short needles and particles. In addition, trace-element analyses revealed two types of spinel from this new deposit: attractive blue stones that showed an average Co concentration of 32 ppm (designated type I), and darker material with distinctly higher Co (averaging 200 ppm; designated type II). The latter showed a beautiful blue colour only when cut into melee-sized stones (i.e. about 2 mm diameter). Based on their trace-element composition, stones from this new deposit near Lukande can be separated from Co-bearing spinels from Vietnam, Sri Lanka and Pakistan. U-Pb dating of surface-reaching zircon inclusions indicates the Tanzanian spinels formed during a late stage of the East African Orogeny, possibly overprinted by the Kuunga-Malagasy orogeny (about 500–570 million years ago).