Certain meteorites, called primitive chondrites, contain microscopic diamonds (Fig. 1, overleaf). These diamonds are of astrophysical as well as min-eralogical interest, because they are thought to have formed before the Solar System condensed. In a new experiment—whose results were reported at a meeting in Dublin in July — two classes of pre-solar diamond have been distinguished, implying that they come from at least two different environments. We believe that these diamonds may be pre-solar because of their peculiar isotopic composition. The anomalous isotope ratios of trace elements in the crystals imply that they cannot have formed from the well-mixed material that makes up the bulk of the Solar System. Instead, at least some of the diamonds must have formed around other stars. They survived collisions and bombardment by cosmic rays in the interstellar medium and in the contracting disk of the early Solar System, and remain preserved in some chondrites. Those that fall to Earth and are collected provide an opportunity to study Stardust in the laboratory.
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