The origin of color in natural C center bearing diamonds

Abstract

The properties of 152 natural diamonds with C centers – detectable by the absorptions at about 1344 and/or 2688cm−1 in the infrared spectra – were analyzed in order to better understand their origin of color. While such diamonds are generally thought to be yellow, type Ib natural diamonds are usually not so, but mainly orange-yellow, orange, brown, ‘olive’ (a mixture of yellow with brown and/or gray with always a greenish component) and mixtures thereof. The only natural diamonds found to be of pure yellow coloration were – with very few exceptions – type IaA diamonds with a very minor Ib component, of cuboid–octahedral growth, often so-called re-entrant cubes. This was verified by the analysis of over 70,000 bright yellow and over 20,000 yellow-orange melee diamonds (i.e. diamonds weighing less than 0.20cts) submitted for testing at the laboratory.In natural type Ib diamonds of octahedral growth the color is strongly influenced by vacancy-related defects that originate mainly from plastic deformation; natural type Ib diamonds of regular octahedral growth generally show distinct deformation-related strain and often some associated color zoning or ‘colored graining’ along octahedral planes. None of the nickel-rich, C-center-containing natural diamonds included in this study showed any specific Ni-related absorption band in the visible range spectrum that had an influence on color.The “olive” to brown color in type Ib diamonds was found to be caused by a combination of continuum absorption with increased absorbance from the NIR to about 480nm plus distinct NV− center absorption.