Rare Natural Bicolor Diamond

Mei Yan Lai and Sally Eaton-Magaña

A 0.63 ct bicolor diamond was submitted to GIA at the 2023 Tucson gem show for scientific examination. Previously submitted to GIA in 2008 for a colored diamond grading report, this tapered baguette-cut natural diamond had two major color zones: Fancy Dark orangy brown and Fancy Dark brown greenish yellow (figure 1). The brown greenish yellow zone had chameleon properties, including a temporary color change upon heating or when left in darkness for an extended period of time (possibly days).  Visible narrow bands with darker color were observed within the brown greenish yellow zone (figure 1). Fourier-transform infrared spectra collected from the two color zones indicated that both were type Ia with comparable nitrogen concentrations. No amber centers or carbon dioxide absorption were detected in either color zone.

Bicolor diamonds are very uncommon, and the colors likely result from a combination of lattice defects. The major causes for yellow coloration in diamond include N3 centers, C centers, H3 centers, and the 480 nm absorption band (C.M. Breeding et al., “Naturally colored yellow and orange gem diamonds: The nitrogen factor,” Summer 2020 G&G, pp. 194–219), whereas brown coloration is typically associated with vacancy clusters produced during plastic deformation (D. Fisher, “Brown diamonds and high pressure high temperature treatment,” Lithos, Vol. 112, Supplement 2, 2009, pp. 619–624). It has been reported that diamonds rich in carbon dioxide might also have a brown color (T. Hainschwang et al., “HPHT treatment of CO₂ containing and CO₂-related brown diamonds,” Diamond and Related Materials, Vol. 17, 2008, pp. 340–351). Ultraviolet/visible/near-infrared spectra collected separately from the two color zones to determine their color origins indicated that both color zones contained the 480 nm absorption band. In addition to this 480 nm absorption band, the brown color zone also showed enhanced absorption extending from the near-infrared region toward the UV region of the spectrum.

Diamonds colored by the 480 nm absorption band generally have multiple growth zones with distinct fluorescence colors excited by deep UV (<230 nm), due to their complex multi-stage growth under changing conditions in the earth’s mantle (Breeding et al., 2020). This bicolor diamond had alternating blue and greenish yellow fluorescence zones (figure 2), similar to the fluorescence patterns in other diamonds with the 480 nm absorption band. The fluorescence zones did not correlate with the major color zoning (i.e., orangy brown and brown greenish yellow) in this diamond. However, the greenish yellow fluorescence bands (figure 2) appeared to correspond to the narrow bands with darker color in the brown greenish yellow color zone (figure 1).

A cluster of orange mineral inclusions was observed beneath the table facet, where the largest inclusion measured 180 μm in maximum dimension (figure 3). The Raman spectrum of the largest inclusion showed the characteristic peaks of pyrope-almandine-grossular garnet, (Mg,Fe,Ca)₃Al₂(SiO₄)₃, at 358, 558, and 917 cm^–1 (e.g., E.M. Smith et al., “Raman identification of inclusions in diamond,” Reviews in Mineralogy and Geochemistry, Vol. 88, 2022, pp. 451–473), indicating the diamond was associated with eclogite host rocks having pyrope-almandine-grossular garnet as a major constituent mineral.

This unique bicolor diamond exhibiting two distinctly different colors demonstrates the variety possible within the natural world.