The electron paramagnetic resonance and electron nuclear double resonance spectra of natural blue diamonds were reported for the first time. The diamonds were shown to contain the NE2 centre, which had been observed mainly in synthetic diamonds and had been proposed to consist of Ni+ plus 3 neighbouring N atoms. A previously unreported electron paramagnetic resonance centre was observed, and was found to have S = 3/2, g = 2.02 and superhyperfine interaction from one N atom. The results were consistent with Ni- on a substitutional site and Ni+ on a fourth-nearest neighbour site. This defect was found exclusively in Argyle blue diamonds.

Electron Paramagnetic Resonance Investigations of Nickel Defects in Natural Diamonds. C.J.Noble, T.Pawlik, J.M.Spaeth: Journal of Physics - Condensed Matter, 1998, 10[50], 11781-93