Lab Notes Gems & Gemology, Summer 2020, Vol. 56, No. 2

Multiple Radiation Stains Suggest Interesting Geological Residency

Troy Ardon and Sally Eaton-Magaña

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Radiation stains on a rough diamond crystal.
 
 
 
 
Figure 1. Radiation staining is visible on opposite sides of the rough diamond crystal. On the upper surface the green stain is closer to the bottom of the image, whereas on the lower surface the green stain is closer to the top of the image. The arrows indicate the direction of displacement, which are in opposite directions on opposite sides of the rough crystal. Field of view 2 mm. Photomicrograph by Troy Ardon.

A rough diamond crystal weighing 4.05 ct was observed with radiation staining on its surface. Radiation staining is thought to occur when radioactive fluids or minerals are adjacent to a diamond crystal in the earth. The radiation imparts damage to the diamond lattice, leaving behind vacancies and interstitial carbon atoms. When initially formed, these stains are green or greenish in color. If they are subjected to heat, the stains will turn to a brownish color. This particular diamond had both green and brown radiation stains on the surface. Nearly all of the stains existed in pairs, one brown and one green, and were of the same shape. This has been reported previously (C.M. Breeding et al., “Natural-color green diamonds: A beautiful conundrum,” Spring 2018 G&G, pp. 2–27), and the proposed mechanism is that the diamond crystal is adjacent to a radioactive substance that imparts a green stain. When the diamond crystal is shifted slightly, the radioactive minerals create a new stain of the same shape but slightly displaced. The diamond was heated before the second set of stains had formed, which turned the initial set of stains brown.

In this rough crystal, the stain pairs are on opposite sides and in opposite directions (figure 1). The shift must have occurred in the direction from brown to green, which means this crystal most likely rotated with respect to its environment. The other explanation is that the host material on either side shifted in opposite directions by roughly the same amount.

Several radiation stains in the 4.05 ct diamond and details of their PL mapping.
 
 
 
 
Figure 2. A portion of one side of the 4.05 ct rough diamond (left; field of view 3 mm) shows the relative placement of several radiation stains that are detailed with PL mapping at right. A: The photographed image of a very dark radiation stain and the corresponding PL map of diamond Raman intensity. B: The image of a brown radiation stain separated from its green counterpart and the corresponding PL map of its diamond Raman intensity. C: An image of a light radiation stain. Its PL map plots the TR12 intensity, instead of Raman intensity as in A and B, which was much higher within the radiation stain than within the colorless section. The PL maps in A and B were collected with 532 nm excitation and plot the diamond Raman peak area at 573 nm, while the map in C with 455 nm excitation plots the TR12 peak area at 470 nm and normalized to the diamond Raman peak area. Photomicrograph by Sally Eaton-Magaña.

To examine the spectroscopic differences between the green and brown radiation stains, we collected photo­luminescence (PL) maps using 532 and 455 nm excitation in confocal mode. We collected data on several of the radiation stains but will focus this discussion on three of them (figure 2). Figure 2A has dark, almost black coloration in the center with dark green and brown color around the periphery. Figure 2B shows green and brown radiation stains but with a colorless section between them, and figure 2C has very light radiation staining.

PL spectra showing features of the radiation stains and a nearby colorless area of the diamond.
 
 
 
 
Figure 3. These 532 nm excitation spectra show differences in features between the brown and green radiation stains imaged in figure 2A and a nearby colorless portion of the diamond. In the colorless diamond, the Raman peak is much larger and shows the NV– peak and a weak GR1. The brown and green stains show much lower diamond Raman intensity, and the peak width is greater within the green stain (inset). Both the green and brown radiation stains also show a radiation-related Raman peak at 1640 cm–1 (583 nm; Eaton-Magaña and Moe, 2016) and comparatively pronounced GR1 peaks. Other unknown and intrinsic features are also present in these spectra.

In the radiation stain imaged in figure 2A, the diamond Raman peak was quite broad and distorted within the radiation stain compared to the surrounding diamond (figure 3, inset). The broadened and distorted Raman peaks are indicators of the radiation damage brought upon these areas and are consistent with prior observations of other very dark radiation stains (e.g., S. Eaton-Magaña and K.S. Moe, “Temperature effects on radiation stains in natural diamonds,” Dia­mond and Related Materials, Vol. 64, 2016, pp. 130–142). The average Raman peak FWHM (full width at half maximum) within the green portion of the radiation stain was ~1.6 nm, while the average width within the brown portion of the radiation stain was ~1.1 nm. For comparison, the Raman width in the colorless sections was ~0.5 nm. The detected Raman intensity was also much lower than that of the surrounding colorless diamond (figures 2A and 3). Natural diamond and irradiation-related features such as TR12 (470 nm), H3 (503.2 nm), and NV– (637 nm) were not detected within this heavily damaged radiation stain. The GR1 peak was detected, though with higher intensity within the green radiation stain (figure 3).

In figure 2B, the PL map is consistent with the visual image and shows the area between the green and brown radiation stains approaching the features of the surrounding colorless diamond. Both figures 2B and 2C show indications of less radiation damage compared to figure 2A. The TR12, H3, and NV– features were detected in both the green and brown radiation stains, and the natural diamond features of H3 and NV– showed higher intensity in the brown. The GR1 and TR12 were slightly higher in the green radiation stains than in their brown counterparts and greater still than in the surrounding colorless diamond (figure 2C). For the radiation stains pictured in figures 2B and 2C, the diamond Raman widths were generally equivalent to those of the surrounding diamond. The PL maps also demonstrated that the boundary of the high GR1 intensity extended laterally ~30 μm beyond the colored radiation stain and into the surrounding colorless diamond; this is consistent with prior estimates of alpha radiation penetration (Eaton-Magaña and Moe, 2016).

In the radiation stains shown in figure 2, the brown portions of the radiation stains displayed features closer to the intrinsic diamond (lower diamond Raman width of the radiation stain shown in figure 2A and its corresponding spectra shown in figure 3, along with higher intensities of H3 and NV– of radiations stains shown in figures 2B and 2C; spectra not shown). These features suggest that the time and temperature that created the transition from green to brown as the diamond shifted to a new position also brought some “healing” from the localized radiation effects detected within the green radiation stains. This sample was interesting scientifically, as it allowed some direct comparison of radiation stain features created by the same point sources.

Troy Ardon is research associate, and Sally Eaton-Magaña is manager of diamond identification, at GIA in Carlsbad, California.