Nickel atomically dispersed in the form of Ni-N-vacancy complexes is detected in selected natural diamonds and its location within them determined. The complexes belong to the 'NE' family of EPR-active centres studied in high-temperature annealed HPHT-grown diamonds synthesised in presence of Ni and N. Some NE centres exhibit characteristic photoluminescence (PL) spectra and by these emissions their presence is identified in natural specimens that are found to possess zones of mixed-habit growth where normal {111}-faceted growth has been accompanied by non-faceted 'cuboid' growth on similar to{100} surfaces. Some prior evidence that the Ni-containing centres reside in cuboid growth sectors is strongly confirmed by PL probing with confocal micro spectrography using excitation by 325, 488 and 514.5 nm laser sources. Other spatially resolving techniques applied included FTIR absorption microscopy, X-ray and cathodoluminescence topography, and PL emission pattern microphotography. Growth structures, lattice defects, nickel occurrence, and nitrogen impurity concentration and aggregation state within polished near-central sections of two contrasting specimens are analysed, one specimen originating from the Snap Lake kimberlite intrusion, NW Territories, Canada, the other from an alluvial source in Sierra Leone. (C) 2003 Elsevier B.V. All rights reserved.