Professional Jeweler Archive: Labs Issue Ruby Alerts

March 2005


Labs Issue Ruby Alerts

Glass fillings raise concerns

By Robert Weldon, G.G.

The American Gem Trade Association’s Gem Trade Lab and the Swiss Gemological Institute (SSEF) have issued alerts to the trade regarding glass-filled rubies. While the practice of filling small pits and fissures in rubies with glass has been known for over a decade, the sudden emergence in the market of many rubies with the treatment has raised concerns.

Rubies and other heat-resistant gems are sometimes treated to conceal surface-reaching fractures, fissures or pits. Melted glass is poured over these pits and fissures and then polished over, resulting in a stone with an even, unfractured appearance. Inexperienced buyers who are unaware of the treatment don’t look for it and can end up paying more for a stone than it is worth. Without proper disclosure of the treatment to potential buyers, the practice is considered deceptive.

SSEF’s chief gemologist, Dr. Henry Hänni, says the treatment seems to be a new variation on a glass theme. In the alert issued by the International Colored Gemstone Association Jan. 10, Hänni says the new glass fillings appear to have a lower melting point than earlier fillings, which means that natural rutile silk inclusions in the rubies remain unaffected. At higher temperatures, the silk dissolves – or partially dissolves – a telltale sign of the treatment. The newly observed gems with glass fillings don’t show these characteristics.

Identification via microscopy is straightforward for gemologists or trained eyes, say the labs. Here are some of the clues:

Surface-reaching fissures/fractures: If the stone is examined in reflected light, minute hairline fissures can be observed breaking the surface. Glass fillings are introduced through these fissures. Glass-filled pits may be observed in reflected light; these areas have a different quality of polish than the surrounding gem.

Gas bubbles: By following the path of the fissure and looking into the depths of the gem, you may observe the glass fillings. These generally contain rounded gas bubble inclusions. If they’re constricted to a smaller fissure, these gas bubbles can become elongated and “squashed.” Because rubies don’t have this kind of inclusion in nature, you should strongly suspect glass filling.

Flash effect: While the refractive index of the glass and the host ruby may be similar, their dispersion is different. Under magnification, the point of contact of these two substances (glass and ruby) within a fissure often leads to a “flash effect,” an optical phenomenon that is manifested in a variety of different colors, including violet, purple, blue and green. This effect can be observed flashing on and off when the gem is rocked and turned under magnification.

If you observe any or all of these clues in a ruby, you should reject it as a natural untreated gem or send it to a gemological laboratory for additional testing. Lab technicians can radiograph a gemstone, perform sophisticated spectroscopy, conduct X-ray fluorescence analysis and use other methods for positive identification of glass-filling treatment in a gemstone.

At first glance, this ruby shows no signs it has been treated. Photo by Robert Weldon.
At 10x magnification, one facet shows a pit into which glass filler has been introduced. There’s an obvious difference in reflection. Even with minor repolishing, glass material wears away faster. Photo by Robert Weldon.

Copyright © 2005 by Bond Communications