X-ray-diffraction studies have been carried out to 255 GPa on various sample and gasket configurations in the diamond-anvil cell. Intense fluorescence emission bands were observed at 793 and 830 nm in some cases which correspond to 360 and 550 GPa on the extrapolated ruby pressure scale. The x-ray measured pressures were only 185 and 201 GPa, respectively. Reasons for this discrepancy between x-ray methods and optical methods at ultrahigh pressures are proposed. © 1988 The American Physical Society.