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Glass Filled Star Rubies

History of Glass Filled Rubies:
Since the last decade, many Thai treaters have been experimenting heat treatment of rough rubies with chemicals such as Lead Oxides, Lead Oxichlorides, Bismuth compounds and other such chemicals which transform into high refractive index glass on heating to a modest temperature of about 900 degrees Centigrade. With passage of time and discovery of new rough ruby sources in Madagascar and Mozambique, the treaters became more experienced and started obtaining dramatic results by which “low quality” rubies were transformed into very “high quality” Rubies. This treatment became famous in Rubies as “Lead-Glass Filling” or “Glass-Filling” Treatment and many laboratories started issuing certificates of these rubies with absolutely new nomenclature so as to perfectly describe the nature of the treatment. 
It was only when the rough Rubies from Andilamena in Madagascar were discovered, that a mass production of these goods began. Since the Andilamena Rubies were highly suitable for this treatment, soon the market was flooded with rubies which were “technically” half-Ruby and half-Glass. Many of these treated rubies were initially passed off as natural unheated Rubies since the low degree of heat necessary for the treatment did not affect the natural inclusions within the gemstone which were the basis of differentiation between “Natural, Unheated “ and “Treated” rubies back then. 
With gradual passage of time, major laboratories and gemmologists started looking closely and established that this treatment can be identified easily based on two paramount characteristic of inclusions. One was the presence of colour flashes in the rubies which could be easily seen on microscope under fibre optic lighting and dark-field illumination. These flashes were as a result of the high dispersion of the Lead Glass inside the Ruby. Another characteristic marker was the presence of numerous rounded gas bubbles trapped in the glass during the cooling process of the treatment. Gas bubbles were supposed to be found only in synthetics prior to this treatment being introduced.

However the game of discovery and detection of treatment did not stop, the treaters became more and more skilled, and the color flashes which were one of the easy characters of identification started disappearing from the new production of Lead glass filled rubies. They also started experimenting with the Bismuth compounds which in many cases gave much better results than Lead. It also fooled many laboratories using ED-XRF (Energy Dispersive X-Ray Fluorescence) to identify lead in the chemical composition of the ruby during testing of rubies. Since no lead was present in the ruby they could not be called Lead Glass Filled by the laboratories which again called for changes in the nomenclature of Laboratory Reports.


Lead Glass Filled Star Rubies
Since the beginning of 2010, a new product again was introduced by the treaters into the market – “Lead Glass Filled Star Rubies”. Prior to this, star rubies were considered to be the safest form of natural rubies since any high temperature treatment would dissolve the rutile silk resulting in the star-effect, and only a moon-effect would remain. Even synthetic star rubies could be easily identified based on their appearance and “super-sharp star”. Since the glass filling of rubies takes place at a very low temperature of about 900 degree centigrade it does not really affect the ruby in any way, and the star remains the way it was prior to the treatment with only the lustre and colour of the ruby increasing.

Such rubies even show the absence of the colour flashes, and in some cases when they are opaque they would even mask the gas bubbles present within the ruby causing a huge ‘Identification Crisis’.


Lead Glass Filled Star Rubies

Lead Glass Filled Star Rubies reportedly from Madagascar



The Process:
The process of glass filling star rubies is very much similar to the glass filling of the faceted rubies. The general procedure is as follows: 
  1. The first step is the pre-shaping of the rough ruby so as to remove major inclusions which could interfere in the heat treatment process and cause unpredictable results. Such inclusions include garnet and other solid crystal inclusions which could melt on heat treatment and explode into the surrounding ruby lattice. This could in some cases even decrease the transparency of the ruby than it was originally.
  2. In the second step the rubies are usually immersed in Hydrofluoric acid for 3-4 hours. The purpose of soaking in Hydrofluoric acid is to dissolve all the silicate impurities in the ruby and it also opens up fissures and cavities.
  3. After removal from Hydrofluoric acid, the rubies are generally neutralised in a basic solution. After this, the rubies are pre-heated from around 500-700 degree Centigrade. Pre-Heat is sort of warming up process before the actual treatment. What it does is that it opens up the cavities of the rubies so that the Lead Glass can properly fill the fractures during the actual process.
  4. In the next step, the rubies are coated with mixture of chemicals which are predominantly Lead Oxides, Silicates, Borax and sometimes oxides of Calcium, Potassium, Bismuth and Sodium. There are several permutations for the proportions of various chemicals used. Certain combinations of chemicals may be well suited for rubies from certain origin but the same may not obtain good results for rubies from other origin. These chemicals are applied to the rubies by either Wet Method or Dry Method. In Wet method, the chemicals are mixed with oil, water or in some cases Sodium Silicate and stirred to form a Paste. This paste is then coated on the surface of the Rubies. In the Dry Method, the chemical mixture is directly coated on the surface of the rubies without application of any fluid.
  5. After the application of the chemicals, the rubies are again heated, this time to around 900 degree Centigrade.  The glass fuses and penetrates all the open fissures and cavities of the rubies increasing their transparency. The time for which the rubies are heated at a constant temperature is called the Soak Time. Soak Time depends on various factors but in general it is 4-8 hours.
  6. The rubies are then cooled gradually and then sorted out to separate the stones processed well from the stones that need reprocessing. Since the rubies are already pre-shaped, they just require polishing into cabochons to display the star effect.
Gemmological Properties of Glass Filled Star Rubies:
Visual Appearance: Glass filled star rubies appear waxy in look, sort of plastic in nature. Opaque gemstones however look like natural star rubies and are difficult to detect by the naked eye. On close inspection with a loupe, it can be clearly seen that these star rubies have an abraded surface with a weaker polish than natural star rubies. The weaker polish is as a result of less hardness than natural star rubies. The colour of star rubies may range from Orangish Pink to Deep Red. Some can even be classified as pink sapphires. The cavities on surface of Rubies invariably have some yellow flux residue (glassy in nature) which is the excess chemicals fused into glass on the surface. The star sharpness is usually identical to that found in natural star rubies.
Optical Properties: The Refractive Index, Pleochroism, Optic Sign, Birefringence, Dispersion are characteristic to that of natural star rubies.
Physical Properties: The Specific Gravity of lead glass filled star rubies is found to be slightly higher than that of natural star rubies due to the lead content. The S.G. of glass filled star rubies is generally 4.00-4.03 while that of natural star rubies is 3.99-4.01.
Internal Features: The inclusions such as color flashes and flattened gas bubbles are consistent with glass filled star rubies as well. The color flashes however are not as visible as they were in glass filled faceted rubies. Since the heat treatment is done at a modest temperature, it does not affect the rutile silk originally present in the stone. As a result, it is also possible to retain even a 12-rayed star after the treatment.
Ultraviolet Fluorescence: The fluorescence of glass filled rubies in short wave light (254nm) is a subdued light red to orangy red similar to iron rich rubies. The fluorescence in long wave ultraviolet light (365nm) is medium to dark red. The fluorescence resembles the iron rich rubies found in Mozambique and other African origins.
Hardness & General Wear: One of the major concerns about the new treatment of lead glass filled star rubies is the durability to the general wear and tear. Since a large proportion of the treated star rubies is lead glass, it causes the overall hardness of the star ruby to reduce. A reduction in hardness affects the stone in many ways, for example it affects the polish of the gemstone and also the abrasion of the surface by dust particles consistently floating in the air. No damage is observed in general cleaning procedures such as steam cleaning and ultrasonic cleaning.
Chemical Stability: Another problem is the decreased chemical stability of the treated star ruby to various acids and bases. Lead glass is highly reactive to Hydrofluoric acid, Aqua Regia, and other strong acids which etch the glass and remove it from the fissures and cavities of Ruby, bringing back the original look of the stone before treatment.
Jewellery Making & Repair Stability: In general, during the setting process, the glass filled star rubies appear to be slightly unstable. Although in general the star rubies did not show any great damage, slight scratches were visible around the prongs. This is a result of the softer hardness of glass filled star rubies. During jewellery repairing procedures such as re-tipping of the prong, if the glass filled star ruby comes in contact with heat from the jeweller’s torch it can cause the melting of lead glass in cavities, damaging the overall appearance of the stone.
Heat Stability: The melting point of lead glass in the star rubies is from 600 to 700 degree Centigrade. Although such a high temperature is not attainable during normal wear, it might be a point of concern duri ng the jewellery manufacturing process. At around 800 degree Centigrade the lead glass starts burbling out of the cavities returning the original fractured look of the stone.




Glass filled star rubies are a great concern to the industry since the techniques of identification, without advanced gemmological testing, are limited. The color flashes which were a very definite feature to identify glass filled rubies are absent in glass filed star rubies. Even the observation of gas bubbles is limited owing to the translucent to opaque diaphaneity. For people not knowing about this treatment, it can be a great challenge to identify glass filled star rubies. It is important that a treatment such as this is properly disclosed by the dealers to the consumers at all levels along with explaining the potential risks and other facts which may affect the permanent nature of the treatment. Almost all reputed gemmological labs have made significant progress in accurately identifying and differentiating glass filled star rubies from natural, untreated star rubies.



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