X-ray diffraction measurements indicate that ruby (Cr3+ doped alpha-Al2O3) transforms to the Rh2O3 (II) structure when heated to temperatures exceeding similar to 1000 kelvin at pressures above similar to 100 gigapascals, in agreement with predictions from ab initio quantum mechanical calculations. The high-pressure phase did not quench upon decompression to ambient pressure, and the occurrence of this phase transformation may affect interpretations of static (diamond-anvil cell) and dynamic (shock-wave) experiments at ultrahigh pressures.