: Gagan Choudhary
(This article was first appeared in the Australian Gemmologist, Vol. 24, No.2, pp 44-45)
The ornamental gem material malachite is known for its striking curved banding in different shades of green. On occasion, however, monochromatic specimens have been seen. At the Gem Testing Laboratory in Jaipur, India, I examined one such specimen, a 9.57 ct cabochon exhibiting chatoyancy (figure 1).
Figure 1: This 9.57 ct saturated green malachite displayed a broad chatoyant band.
Also note the fine bands along the length of the cabochon.
The 13.93 x 10.09 x 5.85 mm oval stone showed a broad chatoyant band when examined with a fibre-optic light source. The band was reminiscent of lower-grade cat’s-eye quartz. In some areas, the specimen had a relatively dull lustre, indicating low hardness. Careful examination revealed the banded pattern (again, see figure 1) typical of malachite. Spot RI was measured at around 1.75, with a birefringence blink down to 1.66, the higher value was not visible (i.e., it went beyond the limit of the refractometer). Such a large birefringence blink is typically associated with carbonate minerals, including malachite. Hydrostatic SG was measured at 3.84. No absorption features were observed with the desk-model spectroscope, and the specimen was inert to both long- and short-wave UV radiation. All of these properties are consistent with malachite.
Malachite commonly forms compact masses of radial fibers that produce botryoidal forms and circular colour banding (see, e.g., R. Webster, Gems, 5th ed., revised by P. G. Read, Butterworth-Heinemann, Oxford, UK, 1994, pp. 351–352). Although this cabochon was not distinctly colour banded, magnification revealed a fibrous structure oriented perpendicular to the direction of the subtle banding (figure 2). This structure was apparently responsible for the chatoyancy of the stone.
Figure 2: A fibrous structure oriented perpendicular to the banding was responsible for the malachite’s chatoyancy. Magnified 30x
FTIR spectroscopy revealed several bands in the 5000–3000 cm-1 region (figure 3). Two bands with twin peaks were seen at around 4520–4320 cm-1 and 4300–4020 cm-1; a steep plateau was observed at 3430 cm-1, and a band at around 3350 cm-1. These absorption features are characteristic of carbonate minerals (e.g., calcite). No features related to polymer or resin was present. Qualitative EDXRF analysis revealed the presence of Cu, but no other element was detected. These analyses confirmed the identification as malachite.
Figure 3: FTIR spectra of this chatoyant malachite revealed absorption features typically associated with other carbonate minerals.
Figure 4: The base of the cabochon displayed a milky sheen (left) that disappeared as the stone was tilted (right).
The base of the cabochon also displayed a distinct milky sheen, which disappeared as the stone was tilted relative to the light source (figure 4). This behavior is typical of cat’s-eye and star gems, and is consistent with the malachite showing a true cat’s-eye effect and not merely a banding effect. During the literature search, no reports of malachite cat’s eye were found, which suggests the rarity of this effect in compact / massive minerals like malachite. The sample was only the second cat’s-eye malachite this author has seen.
All photographs and photomicrographs by Gagan Choudhary