Gahnite: Gemstone Information
Gahnite, a cubic member of the spinel group, forms two series, one with that mineral and the other with the iron aluminate, hercynite. Gahnite is ZnAl2O4 and occurs characteristically as octahedra which may be transparent dark blue or bluish green. Some green crystals have been reported to resemble tourmaline and most cut blue specimens have come from Nigeria where fine examples have been found in the Jemaa district. These stones have a hardness of 7.5–8, SG 4.40–4.59, RI 1.805. Artificial material with similar composition is known (O’Donoghue and Joyner, 2003). Natural gahnite occurs as an accessory mineral in granites and granite pegmatites.