Glass or Corundum, that's the Question Ruby and sapphire are gemstones that are treated extensively by heat since centuries. Untreated stones of high quality have always been rare and highly priced, and the heat treatment permitted a constant availability of attractive rubies and sapphires to satisfy the world's gem markets demand for these materials. In the early 1980's a new practice has been introduced by Thai heat treaters heating corundum embedded in a mixture of a flux (borax or a similar substance) and quartz; with this treatment not only the enhancement of color and removal of certain solid inclusions was possible like in standard heat treatment, but all open fractures were filled with the flux-sio2 glass and due to the lowering of the melting temperature with the flux, some of the corundum started to re-crystalize and the fractures healed artificially. The worst so far in corundum treatments has been introduced in 2004, when rubies appeared in the market with the fractures filled by high high R.I. glass, usually a glass rich in lead. Since this treatment was seen the first time, not one single stone appeared in the lab declared as such, but without exception they were part of scams. The case described here is yet another one, including rubies and sapphires having been represented as standard heat treated material; to the negative surprise of our client the several hundred carats of stones were all heavily lead glass treated and thus of very low commercial value. Background information By Thomas Hainschwang The introduction of silica glass treatment using a flux and quartz during the heat treatment of corundum, (mainly ruby) was the first serious step of the corundum treaters to threaten the market of standard heat treated gems; the healing of fractures by this process permitted to treat material that would likely not survive a standard heat treatment, and stones after treatment were mechanically more stable and by many factors more attractive. In short without the addition of flux-sio2 these stones could not have been used, or only as cabochons. Such treated stones contain glassy residues, sometimes in major quantities (Figure 1). Figure 1. Glassy residues in a flux-sio2 heat treated ruby from MongHsu / Myanmar This much more serious treatment caused the market for heated corundum to react negatively and prices for heated material decreased noticeably. The introduction of the latest treatment, the fracture filling with high refractive index glass, did not shock the market, but opened new unknown possibilities to crocks and criminals: the lead glass treated material is traded for very low prices (commonly a few dollars per carat) and their attractive appearance makes them the perfect items for scams; the material analyzed in the GEMLAB lab was commonly represented as untreated Mogok ruby or standard heat treated material, but the samples were never declared as lead glass treated. This is where the problem occurs for this type of treated corundum: the starting material is commonly unusable for gem purposes and would normally be used at best to be ground into polishing 1
powder or for other industrial applications; the treatment that has been mostly used for stones from Andilamena/Madagascar that are poor in solid inclusions but extremely rich in surface reaching fractures can transform an unusable and virtually worthless stone into an attractive transparent gemstone. The fact that the treatment does not need very high temperatures (about 900 C) results in stones that may appear unheated. The treatment can be easily identified by microscopic observation or if necessary by FTIR, Raman or EDXRF spectroscopy. Description of the latest case This time one of our clients brought several hundred carats of rubies and some carats of sapphires for analysis; the material appeared to be of mediocre quality with some stones of poor and some of good quality (Figure 2). Figure 2. A selection of rubies and sapphires from the large parcel that was represented as standard heat treated material The visual appearance of the rubies was uncommon with a slightly hazy transparency and a somewhat bluish sheen in many samples; this uncommon appearance was the first indication for Pb glass treatment. A glimpse at the samples under the microscope indicated immediately that all the stones were treated with a high refractive index glass, such as lead glass. All stones contained gas bubbles and the glass in the fractures caused a blue interference effect. Figures 3 to 11 show the internal appearance of some of the samples; the images show plenty of gas bubbles, strong blue interference color flashes and whitish residues, plus some natural solid inclusions Figure 3. Gas bubbles in a lead glass treated ruby 2
Figure 4. Gas bubbles, residues and blue interference color in a lead glass treated ruby Figure 5. Gas bubbles and blue interference color in a lead glass treated ruby 3
Figure 6. Gas bubbles, residues and blue interference color in a lead glass treated ruby Figure 7. Gas bubbles and blue interference color in a lead glass treated ruby 4
Figure 8. A very large gas bubble in a lead glass treated ruby Figure 9. Gas bubbles and glass residues in a lead glass treated ruby 5
Figure 10. Gas bubbles and glass residues in a lead glass treated ruby (close-up of figure 9) Figure 11. Flattened gas bubbles and glass residues in a lead glass treated ruby The quantity of fractures present in the corundum was very obvious when the stones were observed in reflected light; the many white reflecting lines seen in figure 12 are all fractures; one can imagine that such a sample was neither attractive nor usable as a faceted gem before the treatment. In figure 13 it can be seen 6
that the glass had a distinctly lower luster and hardness the matte and slightly indented lines are obviously consisting of the low-hardness glass. Figure 12. Reflected light demonstrates the quantity of surface reaching fractures present in the Pb treated corundum Figure 13. Some wide open fractures in this sample were completely filled with the glass; the lower luster and hardness are evident when observing the matte and indented fillings of the fractures. 7
One stone was particularly spectacular, with a very large piece consisting entirely of glass: the low-quality sample shown in figure 14 to 16 had a glass filling that covered more than one third of the largest step-cut facet; in this filling a very large gas bubble was evident, indicating that the filling had a appreciable depth. The filling had an evidently lower luster than the surrounding corundum. Figure 14. A sample with a very large lead glass filling (the silvery reflecting right side of the central step cut facet) Figure 15. A close-up of the filling seen in figure 14, in transmitted light; besides the color difference the very large bubble are more than just indicative for this treatment 8
Figure 16. The filling plus the bubble from figure 15, in dark field illumination. 9
Lead glass treated blue sapphires While by far most lead glass treated corundum is ruby, the other colors of corundum are also being treated this way; here we present one of the rare reports of blue sapphire treated by lead glass; the cabochons included in the parcel of the lead glass treated rubies were all treated the same way. The samples exhibited many fractures and some gas bubbles, usually more flat than spherical. The fractures exhibited an interference effect such as in the rubies, but this effect was much less dramatic and obvious and of yellow to green color. The filled cavities and fractures showed a distinctly lower luster than the corundum. Figures 17 to 21 show the features of such treated samples under magnification. Figure 17. Flattened gas bubble in a lead glass treated blue sapphire Figure 18. yellow interference color in a lead glass treated blue sapphire 10
Figure 19. greenish interference color in a lead glass treated blue sapphire Figure 20. greenish yellow interference color in a lead glass treated blue sapphire 11
Figure 21. Some wide open fractures and cavities in this sample were completely filled with the glass; the lower luster and hardness are evident when observing the matte and indented fillings of the fractures and cavities. Concluding remarks Together with diamond, jadeite and turquoise, corundum is one of the most heavily and extensively treated species in the gem kingdom. In the past 10 years the new color treatments and the new clarity treatments have introduced a new dimension in the corundum market. While before a corundum was either heated or not, today there is the third dimension that includes stones that are heavily treated and that do in some cases not merit the declaration corundum anymore: when high amounts of glass are present in a corundum, then one needs to pose the question where the limit is when one needs to talk about some composite glasscorundum material. While easily identified, the lead glass treatment is unfortunately the perfect tool for scams; many people are unaware that the lead glass treatment exists and that such material is sold for very low prices. Laboratory for Gemstone Analysis and Reports Headquarters Gewerbestrasse 3, FL-9496 Balzers, Liechtenstein Landstrasse 75, FL-9491 Ruggell, Liechtenstein Ph. +423/262 24 64; Fax +423/373 22 43 Ph. +423/373 24 64; Fax +423/373 22 43 laboratory@gemlab.net gemlab@gemlab.net http://www.gemlab.net 12