Gem-quality blue gahnite from Jemaa in central Nigeria has gemmological properties (RI and SG) and a chemical composition indicating relatively pure gahnite (90.7–91.3%). The blue colour is caused by a region of transmission between a system of dominant Fe²⁺- and Co²⁺-related absorption bands centred in the orange-to-green spectral range and an absorption edge in the UV region. The UV edge is strongly temperature dependent and, upon heating of the gahnite to 1000°C and 1400°C, shifts towards the visible range, resulting in blue-green and green colouration, respectively. The green colour component is related to Fe³⁺, whereas the blue hue of the unheated gahnite is due predominantly to Co²⁺.

This article was first published in The Journal of Gemmology, 38(2), 2022, pp. 183-193. A pdf-file of this article is available here.

Authors

Dr. Tom Stephan, Dr. Ulrich Henn, both DGemG & Stefan Müller, M.Sc.

© 2022 by Gem-A