In a given batch more than 30%-40% of polycrystalline, MgO-doped Al2O3 tubes were converted into single crystals of sapphire by abnormal grain growth (AGG) in the solid state at 1880degreesC. Most crystals grew 4-10-cm in length in tubes with wall thicknesses of 1/2 and 3/4 mm and outer diameters of 5 and 7 mm, respectively, and had their c-axes oriented similar to 90degrees and 45degrees to the tube axis. Initiation of AGG was associated with low values of bulk MgO concentration near 50 ppm. The unconverted tubes did not develop centimeter-size crystals but instead exhibited millimeter-size grains. The different grain structures in converted and unconverted tubes may be related to nonuniform concentration of MgO in the extruded tubes. The growth front of the migrating crystal boundary was typically nonuniformly shaped, and the interface between the single crystal and the polycrystalline matrix was composed of many "curved" boundary segments indicative of classical AGG in a single-phase material. The average velocities of many migrating crystal boundaries were quite high and reached similar to1.5 cm/h. The average grain boundary mobility at 1880degreesC was calculated as 2 x 10(-10) m(3) /(N.s), representing the highest value reported so far in Al2O3 and within a factor of 2.5 of the calculated intrinsic mobility. Under similar experimental conditions sapphire crystals did not grow when a codopant of CaO, La2O3, or ZrO2 was added in concentrations of several hundred ppm.