Flux-healed and glass-filled rubies – new challenges
Gagan Choudhary, FGA
In recent times, there has been encounter of rubies treated with flux-healing process, as well as the presence of lead- and bismuth-based glass on surface (as coating), in surface cavities and along the fissures. Detection and disclosure of glassy material in these treated rubies is much more challenging compared to the typical ‘lead-glass filled’ rubies, and hence, leads to disclosure as only ‘flux-healing process’, commonly referred to as ‘heating with residue’. However, identification of ‘glass-filling’ is also dependent on disclosure policy followed by individual gemlab, and hence there is a variation in results when same ruby is tested at different gemmological laboratories.
So-called, “glass-filled rubies” or rubies filled with lead-glass, started to appear in the trade in early 2000, and by 2004 this material flooded the market with some high-quality transparent gems. The main objective of this glass-filling was to enhance clarity of otherwise low-quality, semi-translucent to opaque rubies primarily originating from Madagascar (e.g., McClure, 2006). The unsalable material suddenly became prevalent in the trade at much more affordable prices compared to those of similar quality natural or heated rubies. However, filling of surface cavities of rubies and sapphires using silica-glass is much older and appeared in 1980s itself. Gradually, over the years, lead-glass filling started to appear in rubies originating from other deposits too, including Mozambique, Tanzania and Myanmar (Burma). Like lead, bismuth is another element which has been added to the glass, increasing its refractive index, and occasionally, such bismuth-glass filled rubies have also been encountered.
Prior to these glass-filled rubies, flux-assisted healing of fissures was introduced in [early 1980s see Kane,1984], especially on rubies originating from Mong Hsu (Myanmar) and today also, this treatment is very much prevalent. The treatment process involves heating of rubies at elevated temperatures of ~1850 degrees Celsius in presence of a flux, such as borax. At these elevated temperatures, ruby (Al2O3) surface in contact of molten flux slowly dissolves and during cooling down, [alumina oxide] separates out from the flux and recrystallizes on ruby surface. If recrystallization takes place along the fissures, the build-up of synthetic material will cause the fissures to heal or close, thereby improving the clarity (e.g., Hughes and Galibert, 1998). Therefore, this process is often called as ‘fissure-healing’, rather than ‘fissure filling’. Such treated rubies are commonly disclosed as ‘heated with residue’ by most gemmological laboratories.
Fast forward to 2009-10, elevated quantities of silica were being added to flux (borax) during the fissure-healing process, especially in rubies with wide fissures. This resulted in fissure healing of rubies as briefed above, as well as filling of fissures with silica-glass.
Since mid of 2021, the author has seen numerous samples of rough and faceted rubies (figures 1 and 2) in Jaipur market, which mainly display features associated with ‘flux-healing’, but also contain minor to moderate amounts of lead and bismuth, especially where fissures are wide, or rubies contain surface cavities. In case of rough, glassy material is mainly present as coating.
Figure 1: Rough samples of rubies treated with flux-healing and bismuth-glass filling. Note the glassy coating layer on surface.
Figure 2: Faceted rubies treated with flux-healing and bismuth-glass filling.
In treated rough rubies, the red colour appears much purer and deeper compared to cut and polished samples. This is mainly due to the coating layer of glassy material on the surface. After cutting and polishing, the colour is more towards purple to pink, and not as per the desired colour judged by the rough. This change in colour not only affects the pricing of the final product but also affects the supply chain.
In cut and polished samples of these treated rubies, penetration and presence of glass is significantly affected by the presence or absence of surface cavities or width of fissures in the host ruby. Presence of surface cavities and wide fissures will allow the glassy material to penetrate them, while rubies with no cavities or finer fissures will remain free from this glassy material. As a result of this, few polished samples qualify for ‘glass-filling’, while few for only ‘heating with residues’, though all stones are fashioned from the same rough and same type of treatment.
This becomes an area of concern for the trade, who believing that the rough is only ‘heated with residue’ and price accordingly. However, when a gemmological report discloses these rubies as ‘glass-filled’, price of the finished product drops significantly.
The first and foremost method to detect presence of glass or just heating residues is observation under a microscope, followed by observations under ultra-violet light and then concluded by chemical analyses, usually by EDXRF. In some cases, x-radiography may also be used, although the use of this technique reveals high density of the glass, but not its exact chemical component.
Since treatment is performed at elevated temperatures, higher than meting point of glass or flux, both these materials get deposited on surface of rough rubies on cooling down. This appears as coating of solidified colourless to near colourless melt on the surface (figure 3). This coating often contains gas bubbles and /or other impurities. In one of the rough samples examined by the author, brown patches reminiscing iron-stained films were present, trapped within the coating layer. These patches also enhanced the red component of the rough when viewed in reflected light.
Figure 3: A treated rough ruby crystal with coating of solidified glassy melt and brown patches, similar to iron oxide.
Due to difference in refractive indices of ruby and glassy material, both areas display a difference of lustre. Observation of surface reflections help to detect presence of filled cavities, but identification of glass-type cannot be achieved.
Use of flux during the treatment process leads to recrystallization of alumina or growth of synthetic ruby within the fissures or cavities. The fissures when wide enough allow high volumes of melt to penetrate them leading to growth of synthetic ruby in larger areas. This may be visible as granular texture along the fissure or at their openings or within surface cavities. In the samples studied by the author, few samples (both rough and cut) displayed rows and clusters of tiny grains of recrystallized alumina present within the fissures as well as at their openings (figure 4). When present within the fissures, these granular textures were restricted near to the surface and not deep within.
Figure 4: Recrystallization of molten alumina in the form of synthetic ruby, appears as tiny grains on the surface (top), and resulting into granular texture within the fissures (bottom). These features, however, do not indicate glass-filling. Also note the presence of isolated gas bubble within the cavity in bottom image.
In addition, these recrystallized areas or grains displayed deeper red colour compared to the body colour of the host ruby, suggesting additional supply of chromium during the process of recrystallization (figure 5).
Figure 5: In few samples, recrystallized grains also appeared deeper red compared to the body colour of the host ruby. Such features may also result in deeper body colour of rough ruby samples, which are removed during polishing. Also note the presence of an isolated gas bubble in a cavity (lower part of image).
Fissures of all rough and faceted samples displayed features typically associated with healing; these include interconnected and isolated tubes/channels, droplets, or films (figure 6). These features were present throughout the fissures, suggesting that healing took place all along them.
Trapped Gas Bubbles
In glass-filled rubies, trapped gas bubbles are a common feature, however these were not present in deeper areas of the fissures. Few individual gas bubbles were only visible in surface cavities or near the surface break within the fissures. This suggests that glassy material has not penetrated the complete fissures, as was previously seen in fissures filled with silica-glass or typical ‘lead-glass’ filled rubies.
Presence of high RI (lead or bismuth) glass are usually identified by flash effect along the fissures (figure 7); typically blue, green, pink, orange or yellow colour flashes are seen at a particular direction, which change their colour as the stone is tilted. However, no flash effect was observed in any of these treated rubies. As a result, only microscopic examination is not sufficient to detect the presence or absence of lead or bismuth glass.
Figure 6: All rough and faceted rubies displayed fine fissures, with films (left), droplets and channels (right) – the features typically associated with flux-healing.
Figure 7: Flash effect typically seen in lead- bismuth-glass filled rubies was absent in all these treated rubies.
DiamondView imaging (figure 8, a-d) of rough samples revealed chalky bluish green fluorescence, following the surface coating, while the areas without glassy coating appeared red; this suggested that the bluish green fluorescence is related to glass. Cut and polished samples appeared mostly red with minute areas of bluish green fluorescence, present as patches or veins. The fissures, however, remained orangy-red, commonly associated with flux-healed fissures. These small areas of bluish green fluorescence in polished samples suggested that the glass is present mainly in surface cavities or partially along openings of few fissures.
Figure 8: DiamondView images of treated rough rubies (top, right) displayed strong bluish green fluorescence following the glassy coating. After removing the coating layer, fluorescence is restricted only to surface break, appearing as veins (top, left). Faceted samples display similar fluorescence only in surface cavities and recrystallized areas (bottom, left), while the fissures appear orangy-red, with minute spots of bluish fluorescence (bottom, left), suggesting minor amount of glass.
Conclusive detection of presence or absence of lead- or bismuth-glass is done by chemical analyses, using a EDXRF spectrometer. Rough samples coated with glassy material displayed strong bismuth or lead peaks (figure 9), while the same rough after removing coating layer displayed weaker features. Interestingly, polished samples displayed much weaker peak when randomly tested; however, when the same sample was tested by focussing x-rays on the filled cavity or patch, intensity of bismuth feature increased. This further suggests that bismuth glass has not penetrated throughout the fissure and is restricted only to the surface cavities and shallow depth (i.e., around openings) within the fissures.
Figure 9: EDXRF spectra of a typical glass-filled ruby (gray trace), rough treated ruby with glassy coating (blue trace), rough treated ruby after removing glassy coating (red trace) and cut & polished ruby (black trace). Note the peak intensities highlighted with blue areas. Bismuth content in polished areas dropped down significantly, but still suggests the presence of glass.
Micro-radiographs obtained in multiple directions revealed white lines and spots, corresponding to fissures and cavities, suggesting presence of heavy elements such as bismuth or lead.
Figure 10: X-radiographs of flux-healed and glass-filled rubies displaying denser substance along fissures.
CONCLUSIONS AND DISCLOSURE
Based on the studies conducted, it is inferred that these rubies have undergone a two-step treatment process. Step one included heating of low-quality rubies with flux, such as borax, wherein the fissures present within rubies were partially healed. And due to presence of wide fissures, which could not be healed by the flux, heated rubies were then treated with bismuth or lead based glass, as the second step. Therefore, the glass has only penetrated those wide fissures, that too, only near to their surface breaks or got deposited within the surface cavities.
In absence of flash effect typically associated with glass-filled rubies, detection of glass-filling in these rubies is mainly confined to EDXRF analyses. This becomes a challenge for gemmologists who are either not equipped or do not have access to EDXRF spectrometer, without which these glass-filled rubies cannot be conclusively and correctly identified.
Since the glass is restricted only to surface cavities or near the surface break, presence of lead- or bismuth-glass will vary in polished stones, which is dependent on the degree of polishing resulting in removal of glassy material. Therefore, in some cases, where glass is still present in polished stones, these rubies will be classified as ‘glass-filled’, while in cases where glass is removed during polishing will be classified as ‘heating with residues’.
Kane, R.E., 1984. Natural rubies with glass-filled cavities, Gems & Gemology, 20(4), p.187-199.
Hughes R.W., Galibert O., (1998), Foreign Affairs – Fracture Healing/Filling of Mong Hsu Ruby. Australian Gemmologist, Vol. 20, No. 2, p 70-74
McClure, S. F., Smith, C.P., Wang, W., Hall, H., (2006), Identification and durability of lead glass‐filled rubies. Gems & Gemology. Vol. 42, No.1, p 22 ‐34
All photographs and photomicrographs by Gagan Choudhary
Published on: 29.08.2022