Carlsbad, California – The Gemological Institute of America’s Carlsbad laboratory recently evaluated an 8-carat ruby filled with zinc glass.
The research appeared in a lab note in the Fall 2023 issue of GIA’s quarterly journal, Gems & Gemology, written by Shiva Sohrabi and Amy Cooper.
While a lead glass filling is a common treatment for rubies, even seen in a lab-grown stone, this was the first time graders in the lab had encountered a zinc glass filling.
According to the lab note in Gems & Gemology, the 8.57 carat heart-shaped ruby was originally submitted to the lab for identification.
GIA performed standard gemological tests on the stone and found that it exhibited the characteristics of a ruby, including a refractive index of 1.760-1.769, medium to strong red fluorescence to long-wave UV radiation, and very weak red fluorescence to short-wave UV radiation.
A ruby spectrum was obtained using a handheld spectroscope.
However, fibre-optic illumination revealed several fractures in the gemstone containing a whitish filler and air pockets.
Using reflected light, the analysts further observed that the fractures were less lustrous than the ruby’s host mineral, corundum, confirming the presence of some kind of filler, the lab note said.
While the unidentified filler had visual properties similar to those of other glass fillers in corundum, GIA said it did not exhibit the “blue flash effect” typically observed in lead glass-filled rubies, a characteristic noted in the article “Identification and durability of lead glass-filled rubies” in the Spring 2006 issue of Gems & Gemology.
To determine the nature of the filler, the graders performed a basic chemical analysis using energy dispersive X-ray fluorescence spectroscopy (EDXRF).
“If there was lead in [the stone], which is what they initially suspected, it’s easily detectable by this method,” Shane McClure, GIA’s global director of coloured stone services, told National Jeweler.
The test found no lead or bismuth, but did detect chromium, iron and zinc, which McClure said was “something of an anomaly”.
A more sophisticated analysis, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), was also carried out. This technology uses a spot from a laser to collect a tiny amount of material for analysis via a mass spectrometer, a test that McClure says is more accurate and can identify more elements.
It revealed that the filler in the stone was a silica-based glass doped with zinc.
“As far as we know, zinc has never been documented as an element in any of these glasses that they use to fill rubies,” McClure said.
Glass filling treatment has been used to increase the durability and clarity of heavily fractured rubies since the 1980s, with cavity filling noted and described as early as 1984.
While lead glass fillers are the most popular for rubies, bismuth and cobalt have also been used, as noted in the Spring 2020 issue of Gems & Gemology.
The original filler, silica glass, while quite durable, was easily visible due to its refractive index of about 1.5, which is significantly lower than corundum, an observation cited in the Winter 1984 issue of Gems & Gemology.
On the other hand, the lead glass filler discovered in early 2004 had a higher refractive index, around 1.70, and therefore offered better clarity enhancement.
However, the lead glass was not as durable as its silica-based predecessor and was susceptible to damage even from the standard retipping of a prong.
McClure said that compared to lead glass from the 2000s, which contained about 70 percent lead, today’s lead glass often contains more of the durable silica.
The filler seen in the heart-shaped stone, a silica-based glass with zinc as the main additive, appeared to improve the stone’s durability but not its clarity, the lab note said.
“Usually the purpose is to improve both clarity and durability,” McClure said. “In this particular case, for whatever reason, [the filler] didn’t do much to improve the clarity. It could’ve been poorly filled or damaged afterwards. There’s a number of reasons why that might be the case.”
