Summary: To produce a lattice expansion readily detectable by conventional X-ray photographic means in α-irradiated diamond, a dose of order 10¹⁸ α.cm⁻² is required. Such a dose could be produced by pitchblende in ∼2 × 10⁸ yr. and a previously reported lattice expansion in a natural diamond could therefore have occurred by long-term contact with a radioactive material. α-particle doses greater than ∼10¹⁴ cm⁻² produce observable green colouration on the surface of an initially colour-less diamond.