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 crate
rs(4) and in Cretaceous/Tertiary boundary impact ejecta(5,6); these to
o have most often been interpreted as having formed by shock in the so
lid state(4). Here we report the occurrence of diamond/lonsdaleite pla
tes and cubic diamond in association with silicon carbide, in impact m
elts from the Ries crater in southern Germany. We interpret these occu
rrences 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 fr
om vaporized carbon-bearing rocks, and hence may be used as a reliable
diagnostic tool for hypervelocity impact on Earth. This process may a
lso explain the occurrence of diamonds found in sediments (carbonados(
7)), which may result from the 'heavy bombardment' period of early Ear
th history, rather than from mantle-derived diatremes(8).