Lab Notes Gems & Gemology, Fall 2015, Vol. 51, No. 3

Polished Freeform Topaz Imitating Diamond Rough

Freeform topaz under daylight-equivalent light.
Figure 1. The 13.70 ct topaz under daylight-equivalent light. Note the triangular features on each of the octahedral faces. Photo by Jian Xin (Jae) Liao.

Topaz is one of many near-colorless diamond simulant materials that may be faceted to bear a closer resemblance to diamond. Although most diamond simulants in the marketplace are faceted, every now and then, one comes across what appears to be a near-colorless octahedron diamond rough (see Lab Notes: Fall 1996, p. 205; Fall 1997, pp. 217–218; Fall 2007, p. 250; Fall 2009, pp. 230–231). A near-colorless 13.70 ct stone (figure 1) was recently submitted to the New York laboratory as a rough diamond from an alluvial source. At first glance, even with a well-trained eye, this stone could easily be misidentified. It bore a striking resemblance to diamond rough, with a “well-formed” octahedral shape and trigon-like formations on the surface of each of the faces. To further complicate the identification of this stone, topaz and diamond have the same heft due to overlapping specific gravities: 3.52 (+/-0.10) for diamond, 3.53 (+/-0.04) for topaz. The material was doubly refractive, with a spot RI reading of 1.61, properties consistent with topaz. Final confirmation came from Fourier-transform infrared (FTIR) spectroscopy, which revealed the distinctive topaz absorption spectrum shown in figure 2, with peaks at 4802 and 3650 cm–1 (K. Shinoda and N. Aikawa, “IR active orientation of OH bending mode in topaz,” Physics and Chemistry of Minerals, Vol. 24, No. 8, 1997, pp. 551–554).

Topaz IR absorption spectrum.
Figure 2. Many absorption peaks related to the hydroxide ion in the crystal structure of topaz are detected in this infrared absorption spectrum, including those at 4802 and 3650 cm–1. These have never been documented in any diamond absorption spectrum.

Although topaz can be easily shaped and polished into an octahedral shape, the trigon-like figures observed on the surface require an additional fabrication step and are not typically seen in octahedral-shaped simulants. Intentional material processing steps to mask a stone’s identity remind gemologists of the caution and care that need to be taken when dealing with gemstone identification—even when the identity of the material initially seems obvious.

Akhil Sehgal is a gemologist, and Riccardo Befi is applied gemology manager of colored stones, at GIA in New York.