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

Unusual Glass-and-Quartz Triplet Imitation of Emerald

Biqian Xing, Guanghai Shi, and Wenqing Liu

Download PDF
This suspected imitation was identified as a glass-and-quartz triplet.
 
 
Figure 1. This 11.30 ct sample (18.9 × 14.0 × 8.8 mm) was found to be a triplet composed of colorless glass and quartz held together by green cement. Photo by Biqian Xing.

Recently, an 11.30 ct emerald-cut sample sold as an emerald (figure 1) was provided by an anonymous jewelry manufacturer, who suspected an imitation. It resembled a natural emerald with its vivid bluish green color, inclusion abundance, and a vitreous luster. However, examination identified it as a glass-and-quartz triplet, an occasionally convincing imitation of emerald.

This view of the triplet reveals a nearly colorless pavilion and colorless crown.
 
 
Figure 2. Illuminated with diffused transmitted light with water immersion, the assembled nature of this 11.30 ct triplet was obvious in profile view (top image shows a slight downward view, and bottom image shows a slight upward view). The pavilion is nearly colorless (both top and bottom images) due to the refraction and scattering of the green cement by a large number of inclusions, while the crown is colorless and clean under a certain observation angle (bottom image). Photo by Biqian Xing.
Cracks, fluid inclusions, and partially healed fissures in the pavilion and a maze-like pattern in the cement slice.
 
 
Figure 3. Microscopic observation of the triplet revealed numerous cracks (A), fluid inclusions, and lace-like partially healed fissures (B) in the pavilion. C: The cement slice could be seen under reflected light (as shown by the arrow). D: A maze-like pattern in the cement slice. Photomicrographs by Biqian Xing; fields of view 2.51 mm (A), 10.37 mm (B), 2.71 mm (C), and 1.15 mm (D).

When examined via water immersion in a direction parallel to the girdle plane, one can observe that this triplet was made of two pieces of colorless material (crown and pavilion), with a green cement slice in between (figure 2). Using water for the immersion test to observe the internal characteristics did not corrode the sample and yielded better experimental results. No inclusions were observed in the crown. Microscopic examination revealed two-phase (liquid and gas) inclusions, cracks (figure 3A), and lace-like partially healed fissures in the pavilion (figure 3B). In the middle cement layer, no gas bubbles were observed along the separation plane in the assemblage, unlike doublets previously described (H.A. Hänni and U. Henn, “Modern doublets, manufactured in Germany and India,” Journal of Gemmology, Vol. 34, No. 6, 2015, pp. 479–482; Spring 2019 Lab Notes, p. 92). However, the very thin layer of solidified cement (approximately 10 μm thick; figure 3C) showed a maze-like pattern under the microscope (figure 3D).

This triplet had refractive indexes of 1.515 (crown) and 1.544–1.553 (pavilion) and a hydrostatic specific gravity of 2.59. Its fluorescence under long-wave UV radiation was green for the crown and blue-white for the pavilion (affected by the fluorescence of the cement) and inert for both the crown and pavilion under short-wave UV radiation. No obvious absorption was observed with a handheld portable spectroscope, and no reaction was observed with the Chelsea filter.

FTIR spectroscopy identifies glass and quartz.
 
 
Figure 4. FTIR spectroscopy identified the crown of the triplet as glass (peaks at 1061, 968, 770, and 465 cm–1) and the pavilion as quartz (peaks at 1174, 1140, 1084, 800, 781, 693, 540, 488, and 454 cm–1). Spectra are stacked for clarity.

The EDXRF and FTIR results identified the crown as soda-lime-silica glass (69.97 wt.% SiO2, 15.25 wt.% Na2O, and 8.94 wt.% CaO) with characteristic peaks at 1061 (asymmetric vibration modes of the Si-O-Si network), 770 (symmetric vibration modes of the Si-O-Si network), 968 (stretching vibration of the Si-O non-bridging oxygen group), and 465 cm–1 (Si-O-Si and O-Si-O bending modes) (figure 4) (A. Agarwal and M. Tomozawa, “Determination of fictive temperature of soda-lime silicate glass,” Journal of the American Ceramic Society, Vol. 78, No. 3, 1995, pp. 827–829; S.I. Amma et al., “Specular reflectance (SR) and attenuated total reflectance (ATR) infrared (IR) spectroscopy of transparent flat glass surfaces: A case study for soda lime float glass,” Journal of Non-Crystalline Solids, Vol. 428, 2015, pp. 189–196). The pavilion was identified as quartz, with 99.54 wt.% SiO2 and characteristic FTIR peaks at 1174, 1140, 1084 (antisymmetric stretching vibrations of the SiO4 tetrahedron), 800 (SiO4 symmetric stretching), 781 (SiO4 symmetric stretching), 693 (Si-O-Si bending transition modes), 540, 488, and 454 cm–1 (figure 4) (A. Hahn et al., “Using Fourier transform infrared spectroscopy to determine mineral phases in sediments,” Sedimentary Geology, Vol. 375, 2018, pp. 27–35). Unfortunately, the green cement was too thin to analyze with a micro-infrared spectrometer or Raman spectrometer.

The triplet was a typical imitation of emerald but surprisingly convincing when viewed table-up with the unaided eye. With its extremely thin cement slice, maze-like pattern, and lack of bubbles, even a jewelry manufacturer might mistake it for emerald. This investigation reminds us that the examination of potentially composite stones cannot be ignored.

Biqian Xing and Wenqing Liu are doctoral candidates, and Guanghai Shi is a professor and laboratory director, at the School of Gemology, China University of Geosciences in Beijing.