A method for preparing luminescent diamond particles (e.g., fluorescent nanodiamonds). The method includes irradiating diamond particles with an ion beam and heating the irradiated diamond particles in a non-oxidizing atmosphere at a temperature between 600 and 1000° C. The diamond particles have a diameter of 1 nm to 1 mm and the ion beam has a kinetic energy of 1 KeV to 900 MeV. Also disclosed are luminescent diamond particles prepared by this method and methods of using them.