Abstract: Lumps of a mineral or like material comprising two or more solid phases at least one of which is electrically semi-conductive and of different conductivity and permittivity from the other or others are subjected while immersed in water or other high dielectric medium, to the action of an electrical discharge of high enough potential to ionize the mineral. The discharge so generated crushes the lump. The electrodes between which the discharge occurs are so arranged that the discharge is substantially wholly dissipated in the lump. The invention is especially useful for freeing diamond from kimberlite.