Primitive meteorites contain grains of stardust, i.e. ?pre-solar? grains that formed in the outflows or ejecta of stars. Among these are nanodiamonds with a mean size of ~2.6 nm and an abundance reaching up to ~0.15 % by weight. Isotopic analysis of single diamonds (some 1000 carbon atoms on average) is not feasible, but analysis of large numbers (millions or so) of diamonds reveals the presence of trace noble gases, notably xenon, with an unusual isotopic composition. The latter is reminiscent of the p- and r-processes of nucleosynthesis that are thought to occur during supernova explosions. There are differences in detail, however, which may indicate some unconventional types of element synthesis in stars or modification by secondary processes. Recoil loss from nanometer-sized grains during decay of unstable precursor nuclides has been suggested as an explanation, but experiments we have performed do not support this idea. Astronomical observations indicate the presence of diamonds around young stars, but have not unambiguously identified them in the interstellar medium or around evolved stars. Raman spectroscopy of nanodiamonds from the Allende meteorite has revealed a shift of the 1332 cm-1 diamond peak downward to ~1326 cm-1, but it is not clear whether this is due to origin by shock or a feature caused by small grain size. TEM observations are more in line with a CVD origin. Gases within the diamonds were probably trapped by ion implantation and occupy two different types of sites characterized by different release temperatures.

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