The optical properties of type Ia natural diamond were investigated to 364±9 GPa in a diamond anvil cell. The pressures were measured by x-ray diffraction on tungsten sample using a synchrotron x-ray source. The secondary absorption edge of diamond decreased from its ambient value of 3.7 to 2.5 eV at 364 GPa. The visible diamond luminescence at pressures above 300 GPa is different from the characteristic red luminescence of type Ib synthetic diamond. These are the first optical and structural observations at calibrated static pressures exceeding the pressure at the center of Earth.