The New York lab recently encountered a round brilliant diamond, weighing just over 2 ct, that displayed little if any reaction to long-wave UV except for a small spot confined to the area around the culet (figure 1, left). This small area appeared to have medium fluorescence intensity. While the laboratory determination of fluorescence strength and color is performed in the table-down position, we noticed a very different appearance when the diamond was observed through the crown (figure 1, right). Because of the location of the fluorescent area, it reflected evenly throughout the face-up position andappeared to have higher intensity, at approximately the strong/very strong boundary.
DiamondView imaging revealed that the diamond was cut with its growth zonation parallel to the girdle. We speculate that the area of fluorescence in the rough was larger and more centrally located, indicating that the earlier growth showed more blue fluorescence than the later growth, which was basically inert to UV radiation. With the given orientation and placement of the finished stone in the rough, a part of this fluorescent area remained at the culet in the cut stone, leading to this unusual feature.
Absorption spectra in the infrared region confirmed this to be a type Ia diamond with a high concentration of aggregated nitrogen and spectral features typical of a natural diamond. The N3 defect was detected by the presence of moderate-intensity absorption in the UV-Vis absorption spectrum, which is consistent with this type of diamond. The N3 defect is a well-known cause of blue fluorescence in natural diamonds. Based on past experience, it should be pointed out that the area at the culet with medium-strength blue fluorescence does not necessarily have higher concentrations of the N3 defect. The occurrence of other impurities or lattice distortion could affect fluorescence intensity from the N3 defect as well.
The effect of blue fluorescence on the appearance of D-to-Z diamonds has been debated for more than 25 years. GIA conducted a study on this subject in the 1990s (see T.M. Moses et al., “A contribution to understanding the effect of blue fluorescence on the appearance of diamonds,” Winter 1997 G&G, pp. 244–259). The study showed that blue fluorescence had little to no impact on color appearance or transparency except in extremely rare examples of “overblues.” With this in mind, the 2 ct round brilliant offered an interesting opportunity to revisit the earlier experiment’s face-up observations.We compared the diamond, which was graded “H” color, with nonfluorescent diamonds—GIA color master stones—of similar color. As seen in figure 2, no visual difference was observed in the standard color-grading environment. This example supports the conclusion from the 1997 article that the industry would be better served by considering each diamond on its own visual merits.
John King is chief quality officer, and Wuyi Wang is director of research and development, at GIA in New York.John King