Nano-polycrystalline diamond (NPD)1,2 synthesized by direct conversion of graphite contributes to the technical innovation in precision machining and fabrication of hard materials3. The ultra-high hardness and mechanical strength of NPD arises from the well-sintered nanostructure itself. It prevents the development of micro-cleavage and dislocation movement at grain boundaries, enhancing the bulk strength4, as predicted by the Hall-Petch relation5,6. The production of nano-polycrystalline texture at high P-T is, therefore, a state-of-the-art breakthrough in the development of hard materials and has been applied also for SiO2 (stishovite)7 and Al2O3 (corundum)8. Here, we identified a natural counterpart of NPD, which has similar microtextures and formation mechanisms to those of synthetic one, in diamonds collected from a giant impact crater.
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