January 10, 2005
Labs Issue Ruby Alerts
The American Gem Trade Association's Gem Trade Lab and the Swiss Gemological Institute (SSEF) issued alerts to the trade regarding glass-filled rubies. While the practice of filling small pits and fissures in ruby with glass have been known for over a decade, the sudden emergence of many rubies in the market that are treated this way has raised concerns anew.
Rubies and other heat-resistant gems are sometimes treated to conceal surface-reaching fractures, fissures or pits. These pits and fissures are filled with melted flowing glass 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 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, which means that natural rutile silk inclusions in rubies remain unaffected. At higher temperatures, the silk dissolves or partially dissolves, one of the telltale signs the gem has been tampered with. The newly observed gems with glass fillings do not show these characteristics.
Identification via microscopy is nevertheless straightforward for gemologists or trained eyes, say the labs. Here are some of the clues to look for:
Surface-reaching fissures/fractures If the stone is examined in reflected light, minute hairline fissures can be observed breaking the surface of the stone. It is through these fissures that glass fillings are introduced. Glass-filled pits may also 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, glass fillings may be observed. These generally contain rounded gas bubble inclusions. If they are constricted to a smaller fissure, these gas bubbles can become elongated and "squashed" in appearance. As rubies do not have this kind of inclusion in nature, glass filling should be strongly suspected.
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. It can be observed flashing on and off when the gem is rocked and turned under magnification.
If any or all of these clues are observed within the stone, it should be rejected as a natural untreated gem, or sent to a laboratory for additional tests. Labs can radiograph a gem, perform sophisticated spectroscopy, X-ray fluorescence analysis and other methods for positive identification of glass-filling treatment.
Robert Weldon, G.G.