CNx samples subjected to a high pressure-high temperature treatment were studied by micro-Raman spectroscopy, microhardness, and x-ray diffraction techniques. After treatment the CNx material leads to the formation of some highly ordered diamond crystals, showing an extraordinarily low broadening of the 1332 cm-1 Raman line (Δv = 2.43 cm-1). We propose that the graphite-diamond phase-structural transformation takes place through the formation of rombohedral graphite inclusions in the treated sample and the corresponding effect of puckering of the graphite layers. The observation of the band at ∼1621 cm-1 in the Raman spectra of the matrix surrounding the diamond crystals may be considered as an evidence of the proposed model. © 2000 American Institute of Physics.