Type‐IIb diamonds are partially compensatedp‐type semiconductors at room temperature. Radiation damage makes such diamonds highly insulating. This effect can be exploited to make a fast radiation detector which is sensitive both to light and charged particles. This is done by confining the radiation damage to a narrow band within the crystal. The decay time is less than 3×10−10s after illumination with photons of energy 3.68 eV (337 nm). The fact that ’’counting diamonds’’ can now be manufactured from the relatively abundant type‐IIb diamonds, both natural and synthetic, will be of some interest, since natural counting diamonds are very rare.