Nanotwinning is known as a highly effective approach for strengthening structural materials and impeding the degradation of mechanical properties. Recently a major breakthrough was realized when nanotwinned cubic-BN (nt-c-BN) and diamond (nt-diamond) were successfully synthesized from onion-like nanoparticle precursors under high pressure conditions. Understanding the microscopic origin of the twin boundaries, and the formation of such from onion-like precursors, are therefore critically important and can provide guidance to the production of nt-diamond at a larger scale. A research team has studied the nucleation mechanism of nt-diamond samples using multiple experimental and theoretical methods, including the synchrotron diffraction at HPCAT. By a direct high-pressure high-temperature synthesis of nanotwinned diamond from onion carbon without high-density defects, the team has obtained nanotwinned diamond possessing an exceptionally high Vickers hardness of 215 GPa at 4.9 N. The structural transformation from onion carbon to nt-diamond is shown to be a martensitic process, in which the high-density defects may not be necessary for the formation of nanotwinning, but they do play a role in lowering the onset of the transition pressure. Specifically, the appearance of {111} nanotwinned structure and stacking faults was determined by the characteristics of the onion shells, while the accumulation of the stress due to the sliding of the shells cause the crystal to re-align along the shear direction. These findings not only clarify the direct transformation mechanism from onion-like precursors to nanotwinned diamond, but also have broad implications for further exploration of new materials with exceptional properties. More in H. Tang, et al., “Revealing the formation mechanism of ultrahard nanotwinned diamond from onion carbon”, Carbon, 129, 2018.