Gem News International
Gems & Gemology, Spring 2013, Vol. 49, No. 1

An Opal-Calcite Composite

Gagan Choudhary
GNI 147567
Figure 1. This 8.54 ct cabochon was a composite of brown opal and white calcite. Photo by G. Choudhary.
Composites assembled from opaque to translucent gem materials such as turquoise, chalcedony, and chrysocolla have become quite popular in recent years, as evidenced by the number of samples received for identification at the Gem Testing Laboratory in Jaipur, India. We recently examined a white-brown, translucent to opaque oval cabochon (figure 1) that turned out to be a composite featuring an unusual combination of gem materials.

The specimen weighed 8.54 ct and measured 18.03 × 13.12 × 4.63 mm. At first glance, it appeared to be a rock consisting of some brown and white mineral, but the presence of golden veins ruled out such a possibility, suggesting instead a manmade product. Closer inspection of the cabochon from all sides revealed four distinct areas—a colorless polymer, golden veins and patches, a brown mineral, and a white mineral—all showing a different surface luster (figure 2, left). At the base of the cabochon, individual grains of a white mineral were embedded in the colorless polymer, as well as golden areas comprised of fine flakes, composed of zinc (figure 2, right). These features were consistent with those observed in a composite material (e.g., G. Choudhary, “A new type of composite turquoise,” Summer 2010 G&G, pp. 106–113).

SP13-GNI-14-AB 147430
Figure 2. Under reflected light, the cabochon’s luster varied across four distinct portions (left). Note the polymer vein at the center of the image and the parallel planes and cloudy inclusions at the top of the white portion. Some of the golden areas were comprised of fine flakes embedded in polymer (right). Photomicrographs by G. Choudhary; magnified 24× (left) and 48× (right).
Although the cabochon was recognized as a composite, its components had yet to be identified. Spot RIs of 1.56 and 1.45 were obtained from the white and brown portions, respectively. The white portion also displayed a large birefringence blink, typically associated with carbonate minerals such as calcite, while no blink was observed on the brown portion. Under long- and short-wave UV radiation, the cabochon remained largely inert, except for a weak whitish glow observed on the white portions. Furthermore, magnification of the white portion revealed cleavage planes, liquid films, white cloudy inclusions, and doubling, while the brown portion was milky with fine flaky inclusions.
 
Spot RIs and magnification identified the white portion as calcite and the brown portion as opal, but these tests were not sufficient to prove their identity. Raman spectroscopy of the white portion in the 200–2000 cm–1 range (using 532 nm laser) displayed many sharp peaks at 281, 482, 712, 1086 (the strongest), 1435, and 1749 cm–1; these  peaks are associated with calcite. Raman spectra of the brown portion displayed broad absorptions at around 400–500, 800, and 1000 cm–1; these did not display sharp absorption features other than a peak at 487 cm–1. These absorptions are associated with amorphous materials such as opal (opal-A), and the 487 cm–1 peak is associated with Si-O vibrations (e.g., C.J. Brinker et al., “NMR confirmation of strained ‘defects’ in amorphous silica,” Journal of Non-Crystalline Solids, Vol. 99, 1988, pp. 418–428).
 
Identification of this cabochon as a composite was straightforward, but the components were more difficult to detect. Although Raman spectroscopy identified the materials present, the use of these two substances for making a composite remains unclear.

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