A method of providing a plurality of substantially euhedral diamond grains having a mean size of greater than 5 microns and at most 100 microns, the method including providing a synthesis assembly comprising a source of carbon material comprising graphite, a plurality of seed grains on which diamond material can crystallise, and solvent-catalyst material for promoting the crystallisation of the diamond grains; subjecting the synthesis assembly to a synthesis condition by means of an ultra-high pressure furnace, at which synthesis condition the pressure and temperature are such that the solvent-catalyst material is molten, diamond material can crystallise on the seed grains, and substantial spontaneous nucleation of diamond grains does not occur; the synthesis assembly being arranged such that carbon comprised in the source can migrate through the solvent-catalyst material and crystallise as diamond material on the seed grains at the synthesis condition; the synthesis assembly being subjected to a load and heat cycle comprising three stages, the first stage including applying load onto the synthesis assembly such that the pressure therein is increased at ambient temperature to a first pressure in the range of 1 gigapascal to 5 gigpascals; the second stage including applying heat to the synthesis assembly such that the solvent-catalyst material melts; and the third stage including increasing the load such that the pressure in the synthesis assembly is increased to at least 5.5 gigapascals and achieving the synthesis condition, the synthesis condition being maintained for a synthesis period sufficient for at least 50 per cent of the carbon material comprised in the source to be converted into the plurality of diamond grains; the number of seed grains and the amount of carbon material comprised in the source being selected such that the synthesis condition can be maintained by the ultra-high pressure furnace for the synthesis period; in which the amount of carbon material comprised in the source, the number of seed grains and the synthesis period are selected such that substantially all the carbon material comprised in the source is exhausted by the end of the synthesis period, reducing the pressure and temperature, and recovering the plurality of diamond grains from the synthesis assembly. providing a synthesis assembly comprising a source of carbon material comprising graphite, a plurality of seed grains on which diamond material can crystallise, and solvent-catalyst material for promoting the crystallisation of the diamond grains; subjecting the synthesis assembly to a synthesis condition by means of an ultra-high pressure furnace, at which synthesis condition the pressure and temperature are such that the solvent-catalyst material is molten, diamond material can crystallise on the seed grains, and substantial spontaneous nucleation of diamond grains does not occur; the synthesis assembly being arranged such that carbon comprised in the source can migrate through the solvent-catalyst material and crystallise as diamond material on the seed grains at the synthesis condition; the synthesis assembly being subjected to a load and heat cycle comprising three stages, the first stage including applying load onto the synthesis assembly such that the pressure therein is increased at ambient temperature to a first pressure in the range of 1 gigapascal to 5 gigpascals; the second stage including applying heat to the synthesis assembly such that the solvent-catalyst material melts; and the third stage including increasing the load such that the pressure in the synthesis assembly is increased to at least 5.5 gigapascals and achieving the synthesis condition, the synthesis condition being maintained for a synthesis period sufficient for at least 50 per cent of the carbon material comprised in the source to be converted into the plurality of diamond grains; the number of seed grains and the amount of carbon material comprised in the source being selected such that the synthesis condition can be maintained by the ultra-high pressure furnace for the synthesis period; in which the amount of carbon material comprised in the source, the number of seed grains and the synthesis period are selected such that substantially all the carbon material comprised in the source is exhausted by the end of the synthesis period, reducing the pressure and temperature, and recovering the plurality of diamond grains from the synthesis assembly.