Irradiated Blue Diamond
Blue is a very desirable color for diamond. A rare type of blue to violet hydrogen-rich diamond came from the recently closed Argyle mine in Australia (“Gray-to-blue-to-violet hydrogen-rich diamonds from the Argyle mine, Australia,” C.H. van der Bogert et al., Spring 2009 G&G, pp. 20–37). Prior to the discovery of the Argyle mine, G&G reported on a rare 4.28 ct bluish gray diamond with the 550 nm band feature (R. Crowningshield, “Developments and Highlights at GIA’s Lab in New York,” Fall 1969 G&G, pp. 89–90).
Recently submitted to the New York laboratory for identification was a 0.53 ct pear-shaped diamond graded as Fancy Light blue on GIA’s color grading system (figure 1).
Gemological features typical of natural pink type Ia diamonds were observed under the microscope, including very strong graining or glide planes with a pink color. IR spectroscopy confirmed that this is a type Ia diamond. The unusual graining (haziness) seen in figure 1 contradicts the blue color of this diamond.
A UV-visible absorption spectrum collected from this diamond showed a broad band at about 550 nm, typical for this type of diamond and the pink color attributed to it (resulting from plastic deformation of the crystal lattice during growth or post-growth). A GR1 (general radiation damage) peak at about 741 nm was also observed. This radiation damage is responsible for the green and blue colors observed in some diamonds, both naturally colored and treated (figure 2).
This is a very unusual combination of spectral features in the same diamond. While the strong 550 nm band absorption creating a strong blue transmission window at ~460 nm was responsible for the pink color, the relatively strong GR1 absorption and its side band caused the near-violet color. When mapped with a Raman photoluminescence microscope, this GR1 radiation damage could be seen concentrated on one side of the diamond (figure 3). This concentration of radiation damage, in conjunction with the absorption spectroscopy and gemological features, enabled the confident determination that this diamond had been artificially irradiated to create the desirable blue color. This is a great example of color engineering. Subsequent to this determination, a GIA records query search revealed that this diamond had a previous report with a Fancy Light pinkish purple color grade (figure 4).
