Meteoritic diamond
Diamond found in meteorites, from tiny grains made around other stars to shock- or pressure-formed crystals.
Diamonds in meteorites come in several kinds, with very different stories. The tiniest are nanodiamonds, only a few nanometres across, found in primitive chondrites. Some carry unusual isotopic signatures showing that at least part of them formed before the Solar System, around other stars; they were identified in the late 1980s in meteorites including Allende. Much larger diamonds occur in ureilites, where they sit in carbon-rich seams between silicate grains, and in some iron meteorites, notably Canyon Diablo.
How the larger diamonds formed is debated. Most are thought to have formed when shock waves from impacts converted graphite to diamond. However, some diamonds in the ureilite Almahata Sitta contain inclusions that a 2018 study argued could only form under sustained high pressure inside a large planetary body; this interpretation is still discussed.
You will not see meteoritic diamonds by eye; they are microscopic or very small and usually mixed with graphite. Their presence is often noticed first because ureilites and some irons blunt or damage saw blades. Identification uses electron microscopy, Raman spectroscopy or X-ray diffraction.
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