This paper describes a single-impact study of diamond coatings on tungsten substrates. A gas blast erosion test facility was used to impact 2.38-mm-diameter tungsten carbide spheres onto diamond-coated tungsten at a velocity of 37 m/s. The results show that the impacts result in the formation of ring cracks, whose maximum diameters do not show any significant dependence on the coating thickness. The ring cracks do not agree with the contact diameters as predicted using Hertz' dynamic impact theory, the ring crack diameters being significantly larger than those predicted by the theory. These discrepancies can be attributed to a modified stress field generated by friction between the sphere and the target, as well as plastic deformation of the spheres during impact. A closer agreement with the Hertz theory was seen in the case of subsurface damage: significant delamination was observed adjacent to impact sites on coatings where the depth of maximum shear stress was close to the coating-substrate interface. This finding needs to be borne in mind when designing coated systems for use in erosive environments or applications where impact resistance is required.