corundum, treatments.6, gems

One of the important issues at Tucson this year was the nature of the “fillings” in heat-treated rubies. A major concern was the potential misidentification of a natural surface-reaching inclusion as foreign material (see, e.g., J.L. Emmett, “Fluxes and the heat treatment of ruby and sapphire,” Gems & Gemology, Fall 1999, pp. 90–92). Kenneth Scarratt provided the following information. Since 1984, laboratories and gemologists have identified substances in cavities within ruby and sapphire as “glass” based on visual indicators such as surface luster, bubbles, and evidence of devitrification (see, e.g., R.R. Harding and K. Scarratt, “Glass infilling of cavities in natural ruby,” Journal of Gemmology, October 1984, pp. 293–297; R.E. Kane, “Natural rubies with glass-filled cavities,” Gems & Gemology, Winter 1984, pp. 187–199). Sophisticated techniques have not been used routinely, owing to the high cost of advanced testing and, until recently, the limited quantity of glass-filled rubies. In Bangkok, it has become common practice to use hydrofluoric acid—an aggressive acid that easily dissolves silicates and must be handled with extreme caution—to remove glass residues from rubies after heat treatment. This process is carried out either by the treating company or gem laboratories as an added client service. Typically, rubies are immersed in hydrofluoric acid for a few seconds or minutes to remove glass from surface-reaching cavities. However, in recent years, several instances have emerged in which materials that appeared visually to be glass could not be removed with hydrofluoric acid. These materials shared all the visual characteristics of glass fillings, but their resistance to acid treatment cast doubt on their identity. Consequently, the AGTA Gemological Testing Center in New York has adopted a policy of recording Raman spectra for all “glass-like” substances in ruby cavities. This has been enabled by newer, simplified laser Raman microspectrometers. During implementation, the Center found several fillings that looked like glass under magnification but were not silicate glass. In one case, a cavity featured a plateau of a transparent, slightly undulating, glassy material beneath the ruby’s surface. Raman spectroscopy revealed it was actually corundum with a small area of anatase (a titanium oxide mineral). Both corundum and anatase exhibit sharp peaks at characteristic wavenumbers in their Raman spectra, while true glass does not. This example underscores the importance of caution and proper analytical techniques in identifying substances within ruby cavities. Visual cues alone are insufficient for definitive identification.