Growth of large bipyramidal ruby crystals by the evaporation of molybdenum trioxide flux

Large ruby crystals, exhibiting the typical transparent-red color, have been successfully synthesized by the evaporation of molybdenum trioxide flux. The grown crystals were up to 3.1 x 3.0 mm in size. In the case of 99 mass% of flux evaporation, the crystal sizes were dependent on the mixture (solute + flux) amounts. A sixfold increase in the mixture amount (30 to 180 g) resulted in double or higher increase in the average sizes (1.2 x 1.0 mm to 2.5 x 2.2 mm). The basic form of the grown crystals was a hexagonal bipyramid which consists of twelve well-developed {1123} faces. Furthermore, {0001} faces were also observed in large crystals, besides {1123} faces.