The atomic structure of dislocation cores is decisive for the understanding of plasticity in crystalline solids. The core structure of dislocations in sapphire introduced by high-temperature plastic deformation has been investigated with the use of the negative spherical-aberration imaging technique. The ability of this technique to discriminate oxygen columns from aluminum (Al) columns, combined with reproduction of subtle contrast features by image simulation, leads to a markedly detailed atomic model of the dislocation cores. The partial dislocations are Al-terminated, with electrical neutrality being achieved because half of the Al columns are missing. These partials also undergo core spreading, which results in random occupancy of both tetrahedrally and octahedrally coordinated sites, though Al in tetrahedral coordination never occurs in a perfect crystal. Unusual dislocation core structures may be present in other technologically important nonmetallic solids.