A typical coated diamond has been examined by x-ray diffraction and absorption topography. At the commencement of the coat an abrupt change to fibrous, radial growth occurs. This radial mode of growth gives rise to the general spherical habit of thickly coated diamonds. In the specimen studied the range of misorientation in the fibrous coat was 0·43° about the fibre axis and 0·25° about a normal to the fibre axis. Fibre diameters ranged up to some tens of microns. X-ray absorption contrast produced by the more opaque layers in the coat indicated maximum local impurity concentrations of 0·7% MgO or SiO2, or a few tenths of 1% of a heavier element such as Zr; but the concentration of Fe was less than 0·03%.