Gem News International Gems & Gemology, Summer 2013, Vol. 49, No. 2

Turquoise-Rock Crystal Composite

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This 21.58 ct composite was found to have a top section of natural rock crystal and a base of natural turquoise. Photos by Gagan Choudhary.

Recently examined was a faceted rock crystal backed by a slice of turquoise—an obvious but unusual combination of gem materials. The 21.58 ct oval measured 20.02 × 15.03 × 9.05 mm and was easily identified as a composite due to its color and appearance. From the top, it appeared light greenish blue (see photo, left); from the sides, it exhibited a colorless top and bluish green slice at the base (right).

Further tests confirmed the specimen was made of rock crystal and turquoise, based on its colorless and greenish blue components. The colorless portion displayed an RI of 1.543–1.552, with birefringence of 0.009, while the greenish blue layer displayed a vague shadow edge at around 1.61. Under magnification, the colorless portion was clean and free of inclusions, while the greenish blue base showed some whitish cloudy patches, consistent with those in turquoise and similar materials. As expected, the junction plane contained numerous trapped, flattened gas bubbles without color. Under long- and short-wave UV light, a white to light yellowish glow was visible only along the junction plane, indicative of glue. Under a desk-model spectroscope, the specimen showed a weak absorption at about 430 nm.

FTIR spectra taken by orienting the colorless portion displayed peaks at approximately 3595, 3480, 3380, 3305, and 3198 cm–1—a pattern typically associated with natural rock crystal (e.g., Summer 2011 GNI, pp. 146–147). Raman analysis of the greenish blue base using a 532 nm laser in the 200–2000 cm–1 region revealed a strong peak at about 1040 cm–1, with a number of weaker peaks at approximately 238, 334, 427, 482, 554, 596, 645, and 810 cm–1. These peaks are consistent with those of turquoise in both the lab’s database and the RRUFF database of Raman spectra. No polymer-related peaks were present. In addition, qualitative energy-dispersive X-ray fluorescence (EDXRF) analysis revealed the presence of Al, P, Fe, and Cu, consistent with turquoise. As mentioned, a desk-model spectroscope revealed a weak absorption at around 430 nm, and the same was confirmed using UV-Vis spectroscopy, which displayed a sharp peak at approximately 429 nm (Fe3+) and a broad absorption at about 680 nm (Cu2+), suggesting natural rather than dyed color.

While analysis using both classical and spectroscopic techniques identified this as a composite of natural rock crystal and turquoise, the reason for its creation is still unclear.