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Figure 1. This 30.12 ct piece of faceted glass was sold as rubellite. The elongated straight lines are gas bubbles. Photo by Blanca Mocquet.
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We recently examined a 30.12 ct gem that had been sold as rubellite in Jaipur, India. When observed face up, it convincingly resembled a color-zoned rubellite (figure 1). When observed perpendicular to its width, however, the specimen had a very uneven coloration, with alternating colorless and vivid pink cylinders (figure 2). The sample was singly refractive (1.47 RI) and had a hydrostatic SG of 2.51. It was inert to long-wave ultraviolet radiation and fluoresced strong chalky blue to short-wave UV. With the microscope, one could see elongated bubbles parallel to the colored cylinders. In some cases, the bubbles were so elongated that they reached both sides of the faceted stone, mimicking growth channels. All these gemological properties were consistent with a manufactured glass.
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Figure 2. This side view of the sample reveals that the glass is made up of colorless and vivid pink cylindrical layers. Photo by Blanca Mocquet.
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To determine the exact composition of this glass and look for possible variations between the pink and colorless zones, we analyzed its chemical composition with a JEOL 5800 scanning electron microscope (SEM) equipped with a high-resolution Princeton Gamma Tech IMIX-PTS germanium energy-dispersive detector. The composition was also consistent with a glass. We could not detect any systematic chemical differences between the pink and colorless zones.
To understand the origin of the pink color, we measured the UV-visible absorption spectrum of this glass with a Unicam UV4 spectrophotometer in the 350–800 nanometer range. The main spectral features were a broad band centered at ~540 nanometers and a continuum of absorption regularly increasing from the red to the UV region. A similar broad band has been observed in "ruby glass" of equivalent color, according to our colleague Jean-Pierre Razmoket. Therefore, the coloration was likely due to Mie scattering on submicroscopic metallic inclusions of either gold or copper. We could not observe these features using the SEM because they are likely smaller than the resolution of our instrument. This type of material, in particular the pink central cylinders, is not unlike Venetian glass, which has been widely used for centuries to imitate various gem materials.
Benjamin Rondeau
University of Nantes
Emmanuel Fritsch
Institut des Matériaux Jean Rouxel, Nantes, France
Yves Lulzac and Blanca Mocquet
Centre de Recherche Gemmologique Association (CRG) Jean-Pierre Chenet, Nantes, France
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