Gem News International Gems & Gemology, Fall 2022, Vol. 58, No. 3

Gemological Characteristics of Low-Temperature “Gel-Filled” Turquoise

Quanli Chen, Andi Zhao, Yan Li, Yanhan Wu, and Tianchang Liu

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Rough turquoise produced in Hubei Province, China.
 
 
 
 
 
Figure 1. Untreated rough turquoise material produced in Zhushan County, Hubei Province, China, including one piece (indicated with the dotted line) that was selected to be cut into several square pieces. Photo by Andi Zhao.

In the past decade, low-temperature “gel-filled” turquoise (X. Yating and Y. Mingxing, “Filling material and characteristic of polymer-impregnated turquoise in Anhui Province,” Journal of Gems and Gemmology, Vol. 21, No. 1, 2019, pp. 20–30) has been sold in the Chinese market. This special gel filling treatment is quite different from previous resin-filling or impregnation treatments (L. Liu et al., “Technical evolution and identification of resin-filled turquoise,” Spring 2021 G&G, pp. 22–35). The rough turquoise materials are soaked in a polymer composite gel at –10° to –15°C (without pressure) for filling and enhancement of appearance. According to the information obtained from a turquoise factory in Shiyan city, Hubei Province, the process can be generally summarized as follows:

  1. The rough turquoise material is selected, cut, washed, and dried at about 100°C.
  2. The material is immersed in a polymer composite gel solution, then put in the refrigerator and soaked at –10° to –15°C for 7–10 days.
  3. The turquoise material is removed from the refrigerator and heated from room temperature to 80° at 1°C/min, followed by soaking at 80°C for four hours.
  4. The material is further heated by ramping the temperature from 80° to 120°C at 1°C/min, followed by soaking at 120°C for eight to ten hours.
  5. The turquoise material is then cooled from 120°C to room temperature at 3°C/min.
  6. After removal from the furnace, the gel residue is removed by polishing off the surface layer.
The turquoise and gel solution seen under normal and UV light.
 
 
 
 
 
Figure 2. These images show the untreated turquoise, the gel-filled turquoise, and the gel solution in glassware under normal light (A–C) and under UV light (D–F). Photos by Andi Zhao.

The authors selected a piece of rough turquoise material (figure 1) produced in Zhushan County in Hubei Province, China, and cut it into several square pieces. According to the above treatment process, the turquoise was filled using the gel solution provided by the turquoise factory in Hubei. The material before treatment (N-1) was light whitish blue, with uneven color distribution, and black impurities were locally distributed. After treatment, the material’s color deepened to a more desirable blue-green and became more uniform (figure 2), still retaining the original black impurities. After polishing, the surface of the gel-filled turquoise had a waxy luster and the hydrostatic specific gravity changed from approximately 2.32 to 2.21 due to the lower SG of the gel (approximately 1.00). The turquoise before treatment showed medium-light bluish white fluorescence with an abundance of white spots. After treatment, it showed uniform medium bluish white fluorescence (figure 2) under long-wave UV fluorescence. The turquoise was inert to short-wave UV before and after treatment. The light yellow gel solution showed a strong bluish white fluorescence along the edge of the glassware under long-wave UV and was inert under short-wave UV.

FTIR spectra of the gel, untreated turquoise, and low-temperature gel-filled turquoise.
 
 
 
 
 
Figure 3. FTIR spectra of the gel, untreated turquoise, and low-temperature gel-filled turquoise. The characteristic peaks for gel-filled turquoise are a series of peaks at 2966, 2928, 1727, 1637, 1456, 1405, 1378, 1324, and 1297 cm–1 that are caused by the gel.

Fourier-transform infrared (FTIR) spectra of the gel and turquoise samples (before and after treatment) are shown in figure 3. Typical peaks for turquoise were observed, namely two absorption peaks at 3509 and 3463 cm–1 caused by the stretching vibration of hydroxyls (OH) (Q. Chen et al., “Turquoise from Zhushan County, Hubei Province, China,” Fall 2012 G&G, pp. 198–204). Due to strong hydrogen bonding associated with the hydroxyl groups, the absorption peaks are relatively sharp. The absorption peaks at 3288 and 3077 cm–1 caused by OH stretching of water molecules are relatively flat, while the peak associated with the bending vibration of water is near 1637 cm–1 and the intensity of the absorption peak is relatively weak. There are four absorption peaks produced by the PO4 stretching vibration between 1000 and 1200 cm–1, which appear near 1153, 1006, 1056, and 1010 cm–1. In addition, the gel-filled turquoise showed a weak absorption peak (2928 cm–1) caused by a CH2 stretching vibration and a strong stretching vibration peak (1727 cm–1) caused by C=O (Yating and Mingxing, 2019). A series of weak small absorption peaks also appeared at 1456, 1324, and 1297 cm–1. These spectral peaks are consistent with the FTIR spectra of the gel. The spectral peaks can also be used to distinguish between untreated and gel-filled turquoise. Gas chromotography–mass spectrometry identified the main components of the gel as methyl isocrotonate (C5H8O2, 46.05 wt.%), cyclooctatetraene (C8H8, 35.8 wt.%), and oxirane (C2H4O, 5.93 wt.%).

Quanli Chen, Andi Zhao, Yan Li, Yanhan Wu, and Tianchang Liu are affiliated with the Gemmological Institute, China University of Geosciences in Wuhan.