Lab Notes Gems & Gemology, Spring 2021, Vol. 57, No. 1

Bicolor Rough Diamond Crystals

Rough diamond crystals exhibiting both pink and colorless sections.
Figure 1. These two rough diamond crystals (1.75 ct on the left and 1.44 ct on the right) are rare examples of diamonds showing both pronounced pink and colorless sections. Field of view 10.8 mm (left) and 7.4 mm (right). Photos by Michaela Hariňová (left) and Nathan Renfro (right).

The GIA laboratory in Antwerp regularly receives pink diamond crystals for examination as part of the Diamond Origin Report. This recent service has allowed GIA researchers to study a greater number of rough diamonds in addition to their faceted counterparts. Two crystals, weighing 1.75 and 1.44 ct and both reportedly from Australia, were among those submitted. These were considered quite interesting, as they contained colorless and pink sections with distinct boundaries (figure 1).

The color in the vast majority of naturally pink diamonds is attributed to a broad absorption band at about 550 nm within the visible absorption spectrum. This band generally results from distortion of the crystal lattice from plastic deformation due to stress after crystal growth. However, much remains unknown about the actual formation and configuration of this feature. Therefore, these two crystals provide a unique chance to study natural diamond formation and the origin of pink color in greater detail.

The pink section of both analyzed samples likely experienced great stress in order to undergo the plastic deformation necessary to impart the pink color. The colorless sections were not similarly deformed so presumably they represent a different and later growth event (figure 2).

A distinct boundary between the pink and colorless sections can be seen.
Figure 2. A close-up image of the 1.44 ct diamond crystal shows the boundary between the pink (left) and colorless (right) sections. Photo by Nathan Renfro; field of view 3.16 mm.

Visible-NIR absorption spectra collected from the pink and near-colorless sections of the diamonds show differences in the 550 nm region. Observed in the spectrum for the pink portion is the expected absorption at 550 nm, while the spectrum for the colorless portion shows a lack of absorption (figure 3, left).

Spectra collected from the pink and colorless sections.
Figure 3. Left: Vis-NIR absorption spectra for the 1.44 ct crystal collected from the pink and colorless sections. The 550 nm absorption band is prominently visible within the pink section. The spectra are stacked for clarity. Right: PL spectra with 532 nm excitation of the 1.44 ct crystal show a ~600–730 nm emission band collected from the areas of pink banding, which was not detected from the colorless section—this luminescence band has previously been linked with the 550 nm absorption band. The diamond Raman peaks are scaled as equal.

For the 1.44 ct crystal, we also collected infrared (IR) absorption spectra and photoluminescence (PL) spectra from both the pink and the colorless sections using 455 and 532 nm laser excitation wavelengths. The IR absorption spectra for both sections indicate type Ia diamonds with saturated concentrations of nitrogen. Due to this saturation, it was not possible to determine the total nitrogen content or the ratio of the A and B nitrogen centers, which could have helped in further distinguishing these two sections.

Within the PL spectra, the peak widths for the diamond Raman peak and the H3 peak were comparable in both the pink and colorless sections. Additionally, a PL peak at 676 nm was detected in both sections; although its configuration is unknown, it was documented previously in other rough pink diamonds (E. Gaillou et al., “Spectroscopic and microscopic characterizations of color lamellae in natural pink diamonds,” Diamond and Related Materials, Vol. 19, No. 10, 2010, pp. 1207–1220). The other notable feature detected by PL mapping was a broad (~600–730 nm) emission band (figure 3, right) that coincided with the pink banding revealed by immersion in methylene iodide (figure 4). Its detection is consistent with other pink diamonds and likely related to the 550 nm absorption band (e.g., S. Eaton-Magaña et al., “Comparison of gemological and spectroscopic features in type IIa and Ia natural pink diamonds,” Diamond and Related Materials, Vol. 105, 2020, p. 107784).

Immersion in methylene iodide revealed additional details.
Figure 4. The 1.44 ct crystal was immersed in methylene iodide to reveal additional details. The red arrows within the inset (field of view 3.4 mm) indicate the pink banding. Photos by Troy Ardon.

As natural pink diamonds are quite rare, finding two examples of such bicolor crystals that show these distinct pink and colorless sections is an extraordinary find.

Sally Eaton-Magaña is senior manager of diamond identification at GIA in Carlsbad, California. Paul Johnson is manager of analytics at GIA in New York. Ellen Barrie is supervisor of analytics, and Michaela Hariňová is an analytics technician, at GIA in Antwerp, Belgium.