Blue-coloured gem-quality spinel from Nigeria was analysed by wet chemical methods (using atomic absorption spectrophotometry) and investigated by X-ray diffraction. The results showed it to be gahnite (unit cell dimension a=8.091±0.003 Å) containing 36.7% ZnO, 3.58% FeO, and 0.12% MgO. The spinel has an RI of 1.79 and density between 4.4 and 4.59. Broadening of the n[111] XRD reflections indicates a measure of compositional heterogeneity. The gahnite analyses were compared with compositions of zinc spinels from other parts of the world. The analyses cluster into two distinct groups, Mg-rich spinels of metamorphic origin and Mg-poor spinels (including the Nigerian gahnite) with igneous affinities. Diadochy seems to operate within the zinc spinel structure between (Zn + Mn) and (Fe + Mg).