A lumina (alpha-Al2O3) has been widely used as a pressure calibrant in static high-pressure experiments(1-4) and as a window material in dynamic shock-wave experiments(5-14); it is also a model material in ceramic science. So understanding its high-pressure stability and physical properties is crucial for interpreting such experimental data, and for testing theoretical calculations(15-21). Here we report an in situ X-ray diffraction study of alumina (doped with Cr3+) up to 136 GPa and 2,350 K. We observe a phase transformation that occurs above 96 GPa and at high temperatures. Rietveld full-profile refinements show that the high-pressure phase has the Rh2O3 (II) (Pbcn) structure, consistent with theoretical predictions(22). This phase is structurally related to corundum, but the AlO6 polyhedra are highly distorted, with the interatomic bond lengths ranging from 1.690 to 1.847 Angstrom at 113 GPa. Ruby luminescence spectra from Cr3+ impurities within the quenched samples under ambient conditions show significant red shifts and broadening, consistent with the different local environments of chromium atoms in the high-pressure structure inferred from diffraction. Our results suggest that the ruby pressure scale needs to be re-examined in the high-pressure phase, and that shock-wave experiments using sapphire windows need to be re-evaluated.