Discoloured and pure smalt were investigated by means of X-ray diffraction, electron microscopy and reflectance spectroscopy. It was found that the discolouration of smalt could not be explained in terms of devitrification or recrystallisation. Electron micrographs of replicas show a distinctly attacked grain surface with residues of organic material. Ultramicrotome thin sections of discoloured grains illustrate that an unaltered core is embedded in a material no longer showing the conchoidal fracture and presumably consisting of an organo-metal compound. The reflectance spectrum of discoloured smalt gives evidence that the cobalt ion is in sixfold (octahedral) co-ordination rather than in fourfold (tetrahedral) co-ordination, while the characteristic blue colour of unaltered smalt depends on the purely tetrahedral co-ordination of cobalt. It is therefore concluded that the discolouration reaction may be started by the catalytic influence of cobalt on the oil medium but that eventually the grain surface itself is attacked, involving the oxidation of Co²⁺ to Co³⁺ and a change of co-ordination of the cobalt ions. The reaction proceeds from the grain boundary to the centre and may leave, at an intermediate stage of the process, an unaltered smalt core within the otherwise discoloured grain.