Lab Notes Gems & Gemology, Spring 2024, Vol. 60, No. 1

Cloud Inclusions Causing Fancy Light Blue Color in Diamond

Paul Johnson

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Figure 1. The 2.20 ct Fancy Light grayish blue diamond. Photo by Jian Xin (Jae) Liao.
 
Figure 1. The 2.20 ct Fancy Light grayish blue diamond. Photo by Jian Xin (Jae) Liao.

The most common cause of blue color in diamond is the incorporation of boron into the diamond crystal lattice (type IIb), such as the recently auctioned Cullinan Blue (Summer 2022 Lab Notes, pp. 216–217). Other causes of blue color in diamond are hydrogen-related absorption (C.H. van der Bogert et al., “Gray-to-blue-to-violet hydrogen-rich diamonds from the Argyle mine, Australia,” Spring 2009 G&G, pp. 20–37) and irradiation, either natural or artificial. Artificial irradiation using an electron beam is a common diamond treatment (Winter 2020 Lab Notes, pp. 517–518).

Figure 2. The mid-infrared spectrum of type IIb diamond compared to the 2.20 ct grayish blue diamond, which was type IIa with no boron or hydrogen-related features. Spectra are offset vertically for clarity.
 
Figure 2. The mid-infrared spectrum of type IIb diamond compared to the 2.20 ct grayish blue diamond, which was type IIa with no boron or hydrogen-related features. Spectra are offset vertically for clarity.
Figure 3. The UV-Vis-NIR spectrum of the 2.20 ct grayish blue diamond compared to a UV-Vis-NIR spectrum with typical hydrogen absorbance (notably at ~545 nm) and broad bands at ~700–800 nm, causing the desirable violet to blue colors. The inset shows typical colors associated with this absorbance spectrum. The red trace is also inconsistent with type IIb absorption. With a slight rise to higher energy, a yellow color is expected. Spectra are offset vertically for clarity. Inset photo by Robert Weldon.
 
Figure 3. The UV-Vis-NIR spectrum of the 2.20 ct grayish blue diamond compared to a UV-Vis-NIR spectrum with typical hydrogen absorbance (notably at ~545 nm) and broad bands at ~700–800 nm, causing the desirable violet to blue colors. The inset shows typical colors associated with this absorbance spectrum. The red trace is also inconsistent with type IIb absorption. With a slight rise to higher energy, a yellow color is expected. Spectra are offset vertically for clarity. Inset photo by Robert Weldon.

Recently submitted to the New York laboratory for colored diamond grading service was a 2.20 ct Fancy Light grayish blue diamond (figure 1). Compared to the Cullinan Blue, which was graded as Fancy Vivid blue on GIA’s color grading scale, this diamond was type IIa (i.e., no boron) (figure 2). In addition to the absence of boron-related impurities, this diamond lacked any hydrogen-related impurities (observed in the ultraviolet/visible/near-infrared spectrum in figure 3) (van der Bogert et al., 2009) or any GR1-related color centers, thus ruling out these major causes of blue color in diamond. Instead, the UV absorption for the 2.20 ct diamond was similar to what would be expected for a yellow type Ib (isolated nitrogen) diamond (figure 3).

Figure 4. Fine milky cloud inclusions are responsible for the natural blue color of the type IIa diamond, shown in darkfield illumination. Photo by Paul Johnson.
 
Figure 4. Fine milky cloud inclusions are responsible for the natural blue color of the type IIa diamond, shown in darkfield illumination. Photo by Paul Johnson.

The diamond was heavily included with fine cloud-like inclusions, giving it a milky appearance (figure 4). Clouds, sometimes hydrogen-related, often cause gray, brown, black, or even white color in natural diamonds (S. Eaton-Magaña et al., “Natural-color fancy white and fancy black diamonds: Where color and clarity converge,” Fall 2019 G&G, pp. 320–337). In this case, the cloud micro-inclusions caused Rayleigh light scattering, creating a desirable blue color and not the expected yellow color resulting from a slight rise in absorption from lower to higher energy.

Based on these cloud inclusions, the diamond was given a final color grade of Fancy Light grayish blue, a natural origin of color, and an I3 clarity grade.

Paul Johnson is manager of analytics at GIA in New York.