The amount of wear of neutron‐irradiated diamonds relative to unirradiated ones is compared after a standard grinding against silicon carbide. It is found that the amount of wear increases with increasing irradiation, i.e., the diamonds become softer with increasing irradiation. Measurements of density show that it decreases with increasing irradiation. The decrease of cohesive energy of an irradiated diamond is estimated from the number of broken bonds and the strain energy of the density change. The experiments indicate that dislocation motion is not responsible for the wearing of diamonds.