Examination was made of over 200 diamonds, most of which were of gem quality and many of which weighed only a few milligrams About 25 of them showed good conduction properties when submitted to ionizing radiations For depths of crystal penetration which exceeded a certain amount, all the counting specimens responded to both α-particles and β-particles with a response which was at least qualitatively dependent on the energies of the particles In some cases, however, a marked drop in the counting properties was easily demonstrated with α-particles under conditions of feeble penetration. Even at maximum penetration 5-MeV α-particles produced oscillograph pulses in such crystals which were only about three times as large as those from 1-MeV β-particles. Strong correlation was established between the wavelength of the continuous ultra-violet transmission limit and the magnitude of the electrical counting response of the specimen.