We report the discovery of a novel pathway for the transformation from highly oriented pyrolytic graphite foils into amorphous diamond platelets. This pathway consists of three stages of neutron irradiation, shock compression, and rapid quenching. We obtained transparent platelets which show photoluminescence but no diamond Raman peak, similar to the case of amorphous diamond synthesized from C60 fullerene. Wigner defects formed by irradiation are considered to make a high density of diamond nucleation sites under shock compression, of which growth is suppressed by rapid quenching.