Lab Notes Gems & Gemology, Spring 2021, Vol. 57, No. 1

Green Opal Displaying Aventurescence

Amy Cooper, Ziyin Sun, Philip York, and Aaron Palke

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Pyrite inclusions in a green common opal produce a pleasing aventurescence.
Figure 1. Opal with unusual aventurescence, seen with fiber-optic lighting. Photo by Diego Sanchez.

Recently the Carlsbad laboratory received a variegated dark green and blue partially polished rough stone for identification services. The stone measured 23.64 × 20.13 × 15.52 mm and weighed 23.74 ct (figure 1). A vitreous to waxy luster and bright yellow inclusions were observed.

Standard gemological testing revealed a refractive index spot reading of 1.43 and a specific gravity of 2.15 obtained hydrostatically. These properties were consistent with opal. During microscopic analysis, fine hazy clouds and turbidity were seen within the structure, as well as a reddish brown portion with iron staining. Although no play-of-color was observed, an unusual aventurescence was seen in several areas with fiber-optic lighting while rotating the stone. This unusual phenomenon was caused by the scattering of light from the small, eye-visible yellow inclusions (figure 2). These inclusions were later confirmed by Raman spectroscopy as pyrite.
 

Figure 2. Scattered inclusions in the green opal were identified by Raman spectroscopy as pyrite. Photomicrograph by Nathan Renfro; field of view 1.79 mm.
Figure 2. Scattered inclusions in the green opal were identified by Raman spectroscopy as pyrite. Photomicrograph by Nathan Renfro; field of view 1.79 mm.

LA-ICP-MS analysis was performed on the sample to investigate which chromophore was responsible for its green color. The results showed that the stone contained approximately 3540 ppma Fe. No other chromophoric trace elements were measured in significant quantities, indicating that iron was most likely the cause of the green color. UV-Vis-NIR spectra were collected and used to quantitatively calculate the color of this material (figures 3 and 4, table 1). In figure 3, a broad absorption band centered at about 741 nm produces the green color of the opal. This absorption band is likely related to octahedral Fe2+. Previous publications have shown that iron can produce green color in quartz due to an absorption band centered at 741 nm relating to interstitial Fe2+ (L.B. Hebert and G.R. Rossman, “Greenish quartz from the Thunder Bay amethyst mine panorama (TBAMP), Thunder Bay, Ontario, Canada,” Canadian Mineralogist, Vol. 46, No. 1, 2008, pp. 111–124). Octahedral Fe2+ can strongly absorb red, orange, and some portion of yellow light of the visible spectrum to produce a transmission window in the yellowish green region. In this case, a transmission window around 570 nm results in the green color in the iron opal (solid line in figure 3). The presence of some additional Fe3+ cannot be ruled out and may contribute to the rise in absorption at shorter wavelengths into the UV region.

Figure 3. The spectra of the two types of opal were measured and renormalized for comparison with each other with similar lightness (see corresponding color panels in table 1). The solid line represents iron opal. The broad absorption band centered at 741 nm is likely caused by interstitial Fe<sup>2+</sup>. The dashed line represents chromium opal. The smaller absorption band centered at around 600 nm is caused mainly by Cr<sup>3+</sup>.
Figure 3. The spectra of the two types of opal were measured and renormalized for comparison with each other with similar lightness (see corresponding color panels in table 1). The solid line represents iron opal. The broad absorption band centered at 741 nm is likely caused by interstitial Fe2+. The dashed line represents chromium opal. The smaller absorption band centered at around 600 nm is caused mainly by Cr3+.

Another type of green opal colored by Cr3+ (Winter 2020 Lab Notes, pp. 520–521) was compared with the iron-colored opal (solid line in figure 3). Cr3+ produces a smaller absorption band centered at about 600 nm, leading to a transmission window in the green region around 540 nm that causes a less yellow and more pure green color (figure 3).

The two color panels calculated from the two spectra were also plotted in the CIE L*a*b* 1976 color circle seen in figure 4. This allows for a more accurate comparison between the two color panels using quantitative colorimetric data. In the plot, the hue angle of the iron opal is smaller than chromium opal, resulting in a more yellowish green hue. The chroma of the iron opal is larger than that of a chromium opal with similar lightness, resulting in a more saturated color.

Figure 4. Color panels corresponding to the two spectra were plotted in the CIE L*a*b* 1976 color circle. The color of the iron opal, looking more yellowish and saturated, has a smaller hue angle and larger chroma than the color of the more greenish and less saturated chromium opal. Both calculated color panels have similar lightness for comparison (table 1).
 
Figure 4. Color panels corresponding to the two spectra were plotted in the CIE L*a*b* 1976 color circle. The color of the iron opal, looking more yellowish and saturated, has a smaller hue angle and larger chroma than the color of the more greenish and less saturated chromium opal. Both calculated color panels have similar lightness for comparison (table 1).
 

This green common opal is a clear example of iron contributing to the yellowish green color, in contrast to the more pure green color due to the chromium in the opal previously documented with a slightly grayish green color. Both the yellowish green color and pyrite inclusions are consistent with an iron-rich environment. The pyrite inclusions also add a unique display of aventurescence not typically seen in common opal.

Amy Cooper is a senior staff gemologist, Ziyin Sun is a research associate, Philip York is a gemology specialist, and Aaron Palke is senior manager of research, at GIA in Carlsbad, California.