A relatively new production of red corundum, reportedly from Bangkok, has been offered for sale in recent years. Although the producers claimed that these stones were red diffusion-treated corundum (i.e., a shallow coloration induced by the lattice diffusion of chromium), the surface layer is actually a synthetic ruby overgrowth on natural colorless to near-colorless corundum. The overgrowth layer, after recutting, typically ranged from <0.1 to 0.3 mm thick. This material can be distinguished from red lattice diffusion–treated corundum by the presence of a boundary plane and two different types of inclusion features: those that occur in the natural corundum core and those present only in the synthetic overgrowth. This product demonstrates that as treatment techniques continue to evolve, the distinction between natural, treated, and synthetic corundum may become increasingly difficult to define.