Many papers have been published recently concerning the incorporation of nickel in diamond. In this paper we present results of an annealing study, carried out in the temperature range 1500-1900-degrees-C, of nickel-related optical centres in high-pressure synthetic diamond. We establish the temperature regime in which the well known 1.883 eV and 2.51 eV systems anneal out, and observe the concurrent growth of an array of structure which extends throughout the visible region of the absorption spectrum and gives the previously bright-yellow coloured diamonds a rich golden-yellow colour. By carrying out the annealing sequence on diamonds grown using various solvent-catalysts, we establish the conditions under which the annealed structure is maximized and propose that it involves nickel. Both the content and configuration of nitrogen present in diamond are shown to be critical in determining the stable form of nickel which results.
