Summary: The occurrence of kornerupine in the gem gravels of Ceylon has been established, and specimens are described together with examples of an optically pseudo-uniaxial variety of the mineral. Micro-chemical analyses have been carried out on the new material and on ‘prismatine’ from Saxony, and previously published analyses are reconsidered in the light of new knowledge. The repeat formula of kornerupine approximates to Al₂₀Mg₂₀Si₁₈O₈₆, with considerable replacements of Al and Mg by Fe and Na, and of Si by B and perhaps Al. The boron content is fairly constant, but the iron varies widely. The refractive indices, birefringence, and density increase with the iron content. The methods of chemical analysis are described, and include modifications in the micro-determination of boron.