Abstract: The synthesis of sub-five-nanometer nanodiamonds with high uniformity and purity. Inspired by the formation of natural diamond, iron carbide nanoparticles embedded in iron oxide matrices as the carbon source were used. High-pressure-high-temperature treatment of the precursor yields nanodiamonds with tunable diameters down to 2.13 nm and 0.22-nm standard deviation. The disclosed synthesis procedures also include a method for forming fluorescent nanodiamonds.