Lab Notes Gems & Gemology, Fall 2023, Vol. 59, No. 3

Natural Type IIa Diamond with Unusual Red Fluorescence Distribution

Taryn Linzmeyer and Barbara Whalen

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Figure 1. This 3.58 ct natural type IIa diamond with H color and SI<sub>2</sub> clarity displayed a unique fluorescence pattern when exposed to deep-UV light. Photo by Diego Sanchez.
 
Figure 1. This 3.58 ct natural type IIa diamond with H color and SI2 clarity displayed a unique fluorescence pattern when exposed to deep-UV light. Photo by Diego Sanchez.

The Carlsbad laboratory received a natural type IIa diamond with some unusual characteristics. As part of the standard data collection, the diamond type was determined by Fourier-transform infrared (FTIR) spectroscopy and deep-UV fluorescence images collected using the DiamondView instrument. The DiamondView images revealed predominantly red fluorescence on one side of the diamond and blue fluorescence on the other. This 3.58 ct pear modified brilliant diamond had H color and received an SI2 clarity grade due to cavities, chips, and a feather (figure 1).

Figure 2. 514 nm PL spectra collected on the pavilion of opposite sides of the stone. The spectrum plotted in red corresponds with the red-fluorescing region, and the blue spectrum corresponds with the blue-fluorescing region, with Raman lines at 552 nm scaled as equal. Spectra are offset vertically for clarity. PL analysis spots are circled in the deep-UV fluorescence images. Images by Barbara Whalen.
 
Figure 2. 514 nm PL spectra collected on the pavilion of opposite sides of the stone. The spectrum plotted in red corresponds with the red-fluorescing region, and the blue spectrum corresponds with the blue-fluorescing region, with Raman lines at 552 nm scaled as equal. Spectra are offset vertically for clarity. PL analysis spots are circled in the deep-UV fluorescence images. Images by Barbara Whalen.

Type IIa diamonds typically display a blue fluorescence pattern under deep UV as a result of “band A” fluorescence or the N3 defect in the stone (U.F.S. D’Haenens-Johansson et al., “Synthesis of diamonds and their identification,” Reviews in Mineralogy and Geochemistry, Vol. 88, No. 1, 2022, pp. 689–753). High amounts of nitrogen vacancy (NV) centers can cause red fluorescence in diamonds; however, this is rare in type IIa stones, as they are defined by nitrogen concentrations low enough to be undetectable by FTIR (Summer 2016 Lab Notes, pp. 189–190). The NV centers can be detected in a negative charge state (NV–, zero-phonon line at 637 nm) or a neutral state (NV0, zero-phonon line at 575 nm). Photoluminescence (PL) testing revealed much higher concentrations of both NV centers in the red-fluorescing region of the stone (figure 2). This distribution could have occurred if traces of isolated nitrogen were concentrated in one growth area of the stone.

Figure 3. False-color 532 nm PL map showing the Raman-normalized peak area of NV<sup>0</sup> (left) and NV<sup>–</sup> (right). The maps were compiled from 47,040 spectra. The pronounced gradient in NV centers across the diamond resulted in the different fluorescence color observations (see figure 2 insets).
 
Figure 3. False-color 532 nm PL map showing the Raman-normalized peak area of NV0 (left) and NV– (right). The maps were compiled from 47,040 spectra. The pronounced gradient in NV centers across the diamond resulted in the different fluorescence color observations (see figure 2 insets).

PL mapping using 532 nm laser excitation (figure 3) displayed the relative intensities of these defects, demonstrating the wide distribution of the NV centers in opposite sides of the table and crown facets of this diamond. Analysis of this interesting fluorescence feature demonstrated that nitrogen could have been distributed unevenly in a relatively pure type IIa diamond when it initially formed.

Taryn Linzmeyer and Barbara Whalen are analytics technicians at GIA in Carlsbad, California.