An oxide of aluminum, Al₂O₃; the mineral corundum; an important constituent of clay minerals, Al₂Si₂O₅(OH)₄, determining their suitability for firebrick and furnace linings. Synthetic alumina is used as the feed material in aluminum smelters; it is also used in the preparation of paints called lakes, in dyeing, and in calico printing; in granular form it is used for abrasives and grinding or cutting tools of high tensile strength. Most alumina is made via the Bayer process from hydrated aluminum oxides, as found in bauxite, diaspore, and gibbsite. Aluminum oxide can also be made in an electric furnace by fusing bauxite or corundum. Suitably doped alumina is the feed material for boules of synthetic ruby and sapphire made by the Verneuil flame-fusion process. Fused alumina is crushed and used as an abrasive, a refractory, a heating element for electrical heaters, and as a filtering medium.