Cristobalite Stars and Snowflakes in Devitrified Glass
Recently, the authors examined a 3.04 ct transparent green oval mixed cut stone. Basic gemological testing suggested this stone was glass, which was further supported with Fourier-transform infrared and Raman spectroscopy showing characteristic features of artificial glass.
Although artificial glass is one of the most common gem simulants, microscopic observation revealed a unique inclusion scene. In addition to gas bubbles, many six-pointed star-shaped platelets were observed (figure 1). These star-shaped platelets exhibited a dendritic pattern, which could be seen clearly when observed in brightfield illumination (figure 2). In this lighting environment, the stars more closely resembled snowflakes. Additional inclusions were also observed, taking the form of three-dimensional lattices with spikes at each tip (figure 2). All inclusions were identified as cristobalite by Raman spectroscopy.
Cristobalite, a crystallized form of silica, is a common inclusion in artificial glass formed by devitrification (e.g., E.J. Gübelin and J.I. Koivula, Photoatlas of Inclusions in Gemstones, Volume 1, ABC Edition, Zurich, 1986, p. 430; G. Bosshart, “Cobalt glass as a lapis lazuli imitation” Winter 1983 G&G, pp. 228–231). Devitrification is the process by which an amorphous material changes to a crystalline state. Crystalline inclusions produced by this process can give a natural appearance to glass simulants (e.g., Summer 2018 G&G Micro-World, pp. 230–231). However, this artificial glass, which was likely made to imitate emerald, has a unique beauty that would not exist in natural gemstones.
