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Translucent alumina ceramics have exhibited low mechanical properties and a low in-line transmission of unscattered light (less than fifteen percent) because of their coarse micro-structures (greater than 20 μm). New transparent corundum ceramics avoid these shortcomings and can be manufactured with complex (even hollow) shapes and with a four-point bending strength of six hundred to seven hundred megapascals and a macrohardness HV10 greater than 20 GPa. The in-line transmission of transparent ceramics is close to sixty percent in visible light and approaches the theoretical limit in the infrared range. An even higher visible light transmission (greater than eighty percent at one millimeter thickness) is enabled by new submicrometer spinel, which has a macrohardness of HV10 = 14.5 GPa.