Gem News International Gems & Gemology, Summer 2022, Vol. 58, No. 2

Unusual Cat’s-Eye Omphacite Fei Cui Jade

Xiaoyu Lv, Bowen Zhao, and Xiaoying Lu

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Green omphacite <i>fei cui</i> jade displaying chatoyancy.
 
 
Figure 1. This green omphacite fei cui jade (7.4 × 7.0 mm) displayed chatoyancy under fiber-optic light. Photo by Bowen Zhao.

A round cabochon set in a ring with round brilliant and rose-cut diamonds (figure 1) was recently submitted to the National Center of Quality Inspection and Testing on Gold-Silver Products (NGGC) for examination. The center stone, measuring approximately 7.4 × 7.0 mm in diameter, possessed a vivid green bodycolor and exhibited pronounced chatoyancy with a vibrant green sheen. FTIR analysis (figure 2) revealed that the stone was natural untreated omphacite-type fei cui jade, with a typical fingerprint spectrum corresponding with omphacite, a broad absorption band centered at 3500 cm–1 caused by the hydrous interstitial minerals, and the functional group region showing no absorption of any organic filling material commonly used in bleaching and filling treatment, such as Bisphenol A epoxy resin. Subsequent micro-Raman imaging and spectroscopic analysis confirmed the sample as a fairly pure omphacite aggregate, considering that no other mineral impurity was observed or identified (figure 3). The strongest Raman peaks at 682 cm–1 and 1023 cm–1 were attributed to symmetrical Si-Ob-Si stretching/bending and symmetrical Si-Onb stretching, respectively. Ob refers to the bridging oxygens, while Onb represents non-bridging oxygens in silicon tetrahedra.

IR absorption spectrum of omphacite fei cui jade.
Figure 2. Both the functional group region (left) and the fingerprint region (right) of the sample’s IR absorption spectrum were collected with a diffuse reflectance accessory. A K-K transform was applied to the fingerprint spectrum collected.
Raman shift of sample compared with that of omphacite and jadeite.
 
Figure 3. The sample’s Raman shift confirmed its mineral species. The sample matched well with the RRUFF spectrum for omphacite but differed considerably from the RRUFF spectrum for jadeite. The spectra are shifted vertically for clarity.

Element analysis with energy-dispersive X-ray fluorescence (EDXRF) showed that the sample’s major elements were silicon, calcium, aluminum, magnesium, and iron, consistent with omphacite, whose IMA formula is (Ca,Na)(Mg,Fe,Al)Si2O6). It is well known that the Fe2+-induced grayish bluish green bodycolor is most common among the green omphacite varieties. However, this chatoyant sample was characterized by a relatively high chromium content (Cr2O3 ≈ 0.22 wt.%), producing a more saturated and purer green bodycolor rather than a dull green one. This color feature was confirmed by UV-Vis-NIR spectroscopy, with only a single transmittance band centered at 535 nm appearing in the visible range (figure 4).

UV-Vis-NIR spectrum of the omphacite fei cui jade sample.
Figure 4. Two broad absorption bands centered around ~430 and ~650 nm were formed with the contributions of both iron and chromium. The 692 nm peak convincingly indicated the presence of Cr3+.

Microscopic observation with finely tuned fiber-optic light revealed a parallel-arrayed fibrous texture throughout the sample (figure 5). Meanwhile, a vague honeycomb-like pattern was observed at two specific sides near the girdle (the 12 and 6 o’clock positions of the cabochon in figure 1). Thus, under reflected light the stone presented a chatoyant sheen parallel to the ring band.

A fibrous texture (left) and vague honeycomb-like pattern (right) were observed under fiber-optic illumination.
 
 
 
Figure 5. The omphacite fei cui jade’s parallel-arrayed fibrous texture (left) and vague honeycomb-like pattern (right), shown in fiber-optic illumination. Photomicrographs by Bowen Zhao; fields of view 10.45 mm (left) and 8.81 mm (right).

Based on our current knowledge, this green omphacite fei cui jade was probably from Guatemala. Translucent fei cui jade with such fine texture, vivid green bodycolor, and even color distribution has always been considered of high quality and rarity, but the chatoyancy of this sample offered additional value.

Xiaoyu Lv is a gemologist, and Bowen Zhao and Xiaoying Lu are deputy directors of the gem testing department, at the National Center of Quality Inspection and Testing on Gold-Silver Products (NGGC) in Shanghai.