Author: Gagan Choudhary
(This article was first appeared in Gems & Gemology, Vol. 45, No. 2, pp 151-152)
At the Gem Testing Laboratory in Jaipur, India, I had an opportunity to examine an unusual opaque bluish green cabochon (36.63 ct; figure 1) that had a broad but distinct chatoyant band. The colour, greasy-to-dull lustre, and low heft suggested it was serpentine.
Figure 1: This bluish green cat’s-eye cabochon (36.63 ct) proved to be serpentine.
Standard gemmological testing gave the following results: spot RI—approximately 1.57 with no distinct birefringence blink; hydrostatic SG—2.60; fluorescence—weak yellow to long-wave UV; and absorption spectrum—weak bands in the green (~490 nm) and blue (~460 nm) regions seen with the desk-model spectroscope. In addition, the lustre indicated low hardness, which was confirmed by scratching with a fluorite crystal on an inconspicuous part of the sample. These properties are consistent with those reported for serpentine (e.g., R. Webster, Gems, 5th ed. rev. by P. G. Read, Butterworth-Heinemann, Oxford, UK, 1994, pp. 369–372).
Serpentine is a common ornamental stone that is sometimes used as an imitation of jadeite and nephrite because of its similar aggregate structure and colour appearance. It is usually seen in variable hues of blue, green, and yellow, and comprises species such as antigorite, chrysotile, and lizardite, and varieties such as bowenite, williamsite, and ricolite. Chatoyant serpentine, however, is quite rare. “Satelite,” a fibrous variety exhibiting chatoyancy, has been reported from Maryland and California in the U.S. (Webster, 1994) and from Sichuan Province in China (B. Lu et al., “Infrared absorption spectra of serpentine cat’s eye from Sichuan Province of China,” Journal of Shanghai University, Vol. 9, No. 4, 2005, pp. 365–368).
When the cabochon was examined with a microscope, thin parallel planes were visible. These appeared to be composed of fine films oriented perpendicular to the chatoyant band (figure 2), and were thus responsible for the cat’s-eye effect. In addition, a few scattered brown dendritic crystals and white cloudy patches were present (figure 3); this contributor has previously observed such inclusions in serpentine.
Figure 2: Parallel planes consisting of fine films were responsible for the chatoyancy of the serpentine. Magnified 30x
Figure 3: In addition to the parallel planes, brownish dendritic crystalline inclusions and white cloudy patches were present (lower part of image); these are commonly seen in serpentine. Magnified 45x
Because serpentine is a hydrous material, the FTIR spectrum in the 6000–400 cm-1 range exhibited complete absorption from 4500 to 400 cm-1 and two bands around 5000 and 4700 cm-1 (figure 4). This pattern was similar to those of serpentine samples in our reference database. EDXRF analyses revealed the presence of Mg, Si, Cr, Fe, and Ni, which is consistent with the elements expected to be detected in serpentine.
Figure 4: The FTIR spectrum of the serpentine was dominated by water-related features:
complete absorption below 4500 cm-1 and two broad bands at around 5000 and 4700 cm-1.
This was the first time this contributor has encountered this rare variety of serpentine. The origin of this specimen is not known.
All photographs and photomicrographs by Gagan Choudhary