Yellow Zoning in Pink Diamonds
Recently GIA’s Carlsbad laboratory received a few unusual natural, nitrogen-rich type IaA pink diamonds for color origin and identification service. The pink color of these diamonds is caused by the 550 nm visible absorption band, with the color concentrated within parallel narrow bands known as pink graining (S. Eaton-Magaña et al., “Natural-color pink, purple, red and brown diamonds: Band of many colors,” Winter 2018 G&G, pp. 352–377). In diffused light, they displayed faint yellow color zoning (see above, A and C). Further inspection under long-wave UV revealed strong yellow fluorescence confined to areas with yellow color zoning (see above, B and D).
Photoluminescence (PL) spectra (excited by 514 nm laser) collected from the area with yellow zoning show a broad band centered at ~710 nm. This broad emission band and corresponding strong yellow fluorescence are typically observed in yellow or orange diamonds colored by the 480 nm visible absorption band (C.M. Breeding et al., “Naturally colored yellow and orange gem diamonds: The nitrogen factor,” Summer 2020 G&G, pp. 194–219). These gemological and spectroscopic features suggest that separate color centers produce pink and yellow colors within these diamonds, though the 480 nm visible absorption band cannot be detected with ultraviolet/visible/near-infrared (UV-Vis-NIR) spectroscopy (possibly due to the fact that the yellow color zone is volumetrically small compared to the rest of the diamond, and that the UV-Vis-NIR absorption spectroscopy is a bulk analysis technique).
The mechanism of the formation of these unusual pink diamonds has not been determined and to the authors’ knowledge, such diamonds showing localized regions colored by the 550 nm band and 480 nm band have not yet been documented. Since we know very little about the physics of the 480 and 550 nm visible absorption bands, their co-occurrence within single diamonds might help us to better understand the structures of defects that are related to these features. By paying careful attention to the details of color and fluorescence zoning, more of these unusual rare pink diamonds may be identified.
