SHOCK-PRODUCED diamond and lonsdaleite (the hexagonal polymorph) were first observed in experiments involving explosions’ Several classes of meteorites(2,3) contain microcrystalline diamond aggregates that are thought to be produced by impacts with the Earth or in space. Diamonds have also been found in association with several Russian impact craters(4) and in Cretaceous/Tertiary boundary impact ejecta(5,6); these too have most often been interpreted as having formed by shock in the solid state(4). Here we report the occurrence of diamond/lonsdaleite plates and cubic diamond in association with silicon carbide, in impact melts from the Ries crater in southern Germany. We interpret these occurrences as evidence that these phases can be formed by chemical vapour deposition from the ejecta plume of an impact crater. It follows that cubic diamond and silicon carbide may be formed at any impact site from vaporized carbon-bearing rocks, and hence may be used as a reliable diagnostic tool for hypervelocity impact on Earth. This process may also explain the occurrence of diamonds found in sediments (carbonados(7)), which may result from the ’heavy bombardment’ period of early Earth history, rather than from mantle-derived diatremes(8).