Mechanical tests have been performed on sapphire doped with four different concentrations of chromium at temperatures between 800 and 1,000 degrees C. Samples were cut to favor rhombohedral twinning, which is the softest deformation mode of sapphire that inevitably leads to the failure of this material. We have investigated the effect of Cr doping on the rhombohedral twinning. The mechanical test results show that Cr atoms inhibit rhombohedral twinning, which appears to be athermal in the temperature range studied. An increase in the twinning-resolved stress is clearly observed. For 9,540 ppm Cr, the twinning-resolved stress becomes about three times higher than for undoped sapphire, potentially increasing its technological applications in the industry. The examination of lateral surfaces indicates that Cr atoms neither have influence on the twin width nor on the rhombohedral-twinning propagation. All these results have been properly explained within the model of Geipel et al.