Abstract In an investigation into the fracture properties of thin-sectioned corundum crystals, it was noticed that microcracks in electrically-fused corundum obey a hexagonal symmetry in spite of the rhombohedral symmetry of the corundum structure. This fact suggests that the corundum crystals consist of several twins, which have the same optical direction and which cannot be distinguished microscopically. The X-ray Laue back-reflection and etching techniques were applied to commercial 16-mesh grains of electric furnace-fused alumina abrasives. Corundum crystals grown over the crystal size of hundreds of μ to 1 mm. in electric-furnace fusions were found to be generally composed of several penetration twins with a twinning plane (0001). The twins are considered to be growth twins arising in the cooling process. The impurity composition and heat treatment have no marked effect on twinning. The twin junction is highly stable both mechanically and chemically. Cleavage cracks generated in individual crystals are stopped at the twin boundary.