SPHERICAL particles of carbon consisting of concentric graphitelike shells (’carbon onions’) can be formed by electron irradiation of graphitic carbon materials(1,2). Here we report that, when such particles are heated to similar to 700 degrees C and irradiated with electrons, their cores can be transformed to diamond. Under these conditions the spacing between layers in the carbon onions decreases from 0.31 in the outer shells (slightly less than the 0.34-nm layer spacing of graphite) to about 0.22 nm in the core, indicating considerable compression towards the particle centres. We find that this compression allows diamond to nucleate-in effect the carbon onions act as nanoscopic pressure cells for diamond formation.