gems, spinel, gahnospinel, spinel inclusions

Zinc-bearing gem spinels, traditionally termed gahnospinels or zincian spinels, from Sri Lanka are re-examined to establish relationships between chemical composition and physical properties and to assess their distinction from ordinary spinel. The investigation extends earlier work that identified unusually high refractive indices and densities in Sri Lankan magnesium–zinc spinels but provided a chemical analysis for only one specimen. The material examined consists of transparent, faceted blue, bluish violet and violet stones, mostly selected from ordinary commercial parcels attributed to Sri Lanka; precise source localities are unknown. Electron microprobe analyses identify intermediate compositions dominated by spinel, MgAl₂O₄, and gahnite, ZnAl₂O₄, with subordinate hercynite, FeAl₂O₄. Calculated end-member proportions span 45.3–95.1 mol% spinel, 0.2–50.4 mol% gahnite and 1.7–5.4 mol% hercynite. The investigated specimens have refractive indices of approximately 1.715–1.752 and densities of 3.60–4.05 g/cm³, consistent with the ranges established by previous observations. Both refractive index and density increase with the zinc component, while smaller variations in hercynite content also influence these properties. The results support the earlier interpretation of a continuous spinel–gahnite solid-solution series without evidence of a miscibility gap. In contrast to their distinctive physical constants, the stones show no significant differences from ordinary Sri Lankan gem spinels in colour, visible absorption spectra or inclusion characteristics. The discussion attributes the principal visible absorption bands of natural blue spinel to transitions involving tetrahedrally coordinated Fe²⁺, explaining why colour alone does not reveal zinc enrichment. Common internal features include small particles arranged in straight stringers or fingerprint-like patterns, individual octahedral inclusions sometimes accompanied by liquid feathers, and mineral inclusions of varied shapes. In two specimens, inclusions exposed at the gemstone surface were identified as apatite by microprobe analysis. The study therefore confirms that elevated density and refractive index reflect compositional variation towards gahnite, while appearance, absorption spectra and routine inclusion observations cannot reliably separate these zinc-bearing stones from ordinary Sri Lankan spinel.