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We have conducted high pressure–high temperature (HPHT) diamond synthesis experiments at the conditions of growth of superdeep diamonds (10–20GPa), equivalent to the transition zone, using MgCO₃ carbonate (oxidising) and FeNi (reducing) solvent catalysts. High rates of graphite–diamond transformation were observed in these short duration experiments (20min). Transformation rates were higher using the metallic catalyst than in the carbonate system. High degrees of carbon supersaturation at conditions significantly above the graphite–diamond stability line, led to a high nucleation density. This resulted in the growth of aggregated masses of diamond outlined by polygonised diamond networks, resembling carbonado. Where individual crystals are visible, grown diamonds are octahedral in the lower pressure experiments (≤10GPa in MgCO₃ and ≤15GPa in FeNi) and, cubo-octahedral at higher …