We report energy dispersive x-ray diffraction studies on nanocrystalline hexagonal graphite samples (average grain size=12 nm) in a diamond-anvil cell to 65 GPa at room temperature. A structural phase transition to a hexagonal diamond phase beginning at 15 GPa is completed at 55 GPa, and is reversible on decompression. The x-ray diffraction studies were followed by nanoindentation hardness measurements on the pressure treated samples. The obtained hardness values are in the range of 1–2 GPa. Unlike fullerenes, the room-temperature compression of nanocrystalline graphite to 65 GPa did not produce a superhard carbon material. © 2002 American Institute of Physics.