Lab Notes Gems & Gemology, Fall 2017, Vol. 53, No. 3

Unusual Bodycolors of
Precious Opal

Precious opals with unusual bodycolor.
Figure 1. Both of these precious opals show an unusual and attractive bodycolor. The opal on the left is a 6.71 ct transparent pink cushion brilliant. On the right is a 2.37 ct transparent orange round brilliant. Photo by Robison McMurtry.

Two faceted loose stones with unusual bodycolors (figure 1) were recently submitted to the Carlsbad laboratory for an identification report. The first, a 2.37 ct transparent round brilliant, showed a weak to moderate play-of-color with irregular color patches and brushstrokes. It exhibited an attractive orange bodycolor (similar to the hue of some padparadscha sapphires) and a light degree of haziness. Careful microscopic examination revealed turbid orangy clouds and waves of coarse particles. The larger specimen, a 6.71 ct transparent cushion brilliant, also displayed a weak to moderate play-of-color but more of a pink bodycolor. A similarly hazy appearance from clouds of orangy red particles and tiny black inclusions was also observed (figure 2).

Orangy red particles in pink opal.
Figure 2. The pink opal contained clouds of tiny orangy red particles. Photo by Nathan Renfro; field of view 2.88 mm.

Standard gemological properties were consistent with opal. The orange and pink stones both had a specific gravity of 2.03, with refractive indices of 1.442 and 1.445, respectively. Both were inert to long-wave and short-wave UV light. No indication of clarity enhancement, color treatment, or dye was observed in either. The two stones were therefore identified as natural opal with natural color. 

A single drop of water placed on the surface of each stone was observed under direct transmitted light with the microscope in brightfield mode. The opals showed no indication of porosity and did not absorb the water, suggesting they were not hydrophane material (Fall 2013 Lab Notes, pp. 175–176). Crazing was not seen in either opal.

Advanced gemological testing with energy-dispersive X-ray fluorescence (EDXRF) showed properties consistent with natural opal (SiO2· nH2O), but also the presence of Fe, most likely from the clouds of particles. 

While the exact cause of these pink and orange bodycolors is unknown, the minute inclusions might be influencing the perceived color. 

Our gemological observations confirmed the natural origin. However, further study on additional samples would be required to better understand the cause of color in these unique opals. They were reported to be from Mexico, but the author could find no reference consistent with this material.

Jonathan Muyal is a staff gemologist at GIA in Carlsbad, California.