Gem trade fairs provide an excellent opportunity to screen gem materials currently in the market. During the recent BaselWorld fair, the SSEF Swiss Gemmological Institute received a ruby necklace for an identification report. The necklace consisted of 41 translucent “raspberry” red rondelles with a total weight of 290 grams (figure 1). A microscopic inspection revealed the presence of undamaged inclusions such as rutile and negative crystals, indicating that the beads had not been subjected to high-temperature heat treatment.
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Figure 1. All of the ruby rondelles in this necklace (13–23 mm in diameter) proved to be lead-glass filled. Photo by H. A. Hänni, © SSEF Swiss Gemmological Institute.
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However, further examination with the microscope showed evidence of glass filling, such as faint bluish flashes and gas bubbles throughout the beads. In addition, many fractures, twin planes, and cavities were filled with a glassy substance. The presence of the undamaged natural inclusions was not surprising, since modern fissure treatment in rubies can be done with lead-bearing glass at lower temperatures than are commonly used with the traditional borax process. Although the yellowish color in the larger cavities that is characteristic of lead-glass filling was not evident, analysis of the rondelles with EDXRF spectroscopy did reveal the presence of lead in addition to the expected aluminum, chromium, iron, and gallium.
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Figure 2. These semitranslucent ruby crystals from an East African source are typical of the material used for lead-glass filling. Photo by H. A. Hänni, © SSEF Swiss Gemmological Institute.
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While in Kenya recently, this contributor had the chance to see large amounts of rough Tanzanian corundum that was intended for lead-glass treatment in Thailand (figure 2). The tabular crystals were closer to pink sapphire than ruby, but filling of porous material usually causes an increased saturation of the color.
Henry A. Hänni
SSEF Swiss Gemmological Laboratory
Basel
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