  In this paper, the controlled production of high quality metal-free diamond
nanoparticles is demonstrated. Milling with tempered steel is shown to leave
behind iron oxide contamination which is difficult to remove. Milling with SiN
alleviates this issue but generates more non diamond carbon. Thus the choice of
milling materials is critically determined by the acceptable contaminants in
the ultimate application. The removal of metal impurities, present in all
commercially available nanoparticles, will open new possibilities towards the
production of customised diamond nanoparticles, covering the most demanding
quantum applications.
