Extended Data Fig. 1: Optical images (a), XRD patterns (b) of ND/DMG composite samples synthesized at nominal synthesis temperatures (that is 1050˚C, 1100˚C and 1150˚C) under pressure of 25 GPa with heating duration of 1 h and Rietveld refinement analysis of samples recovered from 25 GPa and 1050 °C with heating durations of 30 min (c) and 1 h (d).

Sample1, Sample 4 and Sample 5 are recovered from 25 GPa, 1050 °C; Sample 2, Sample 6 and Sample 7 are recovered from 25 GPa, 1100 °C; Sample 3, Sample 8 and Sample 9 are recovered from 25 GPa, 1150 °C. Sample 1, 2 and 3 correspond to the Composite-1, Composite-2 and Composite-3 in the main text, respectively. The contents of diamond and disordered multi-layer graphene in the composite were obtained from Rietveld refinement. The difference of diamond content in the samples obtained at each synthesis temperature is no more than 3%. Cyan bars: diamond diffraction peak positions, orange bars: diffraction peak positions of graphite with a reduced interlayer spacing. With the increase of heating duration from 30 min to 1 h, the content of diamond in the composite increases from 9% to 17%.