BUYER BEWARE!!!!!!!!!!!!!!! Lead Glass Filled Rubies Another common treatment which is increasingly frequent is the enhancement of rubies with lead glass. The lead-rich content creates a high refractive index glass, which was first used to fill drill-holes and fissures in diamonds during the 1980s. The use of lead glass to fill fissures in gemstones is a more recent process and lead-glass-filled rubies have only become common since 2004. Glass filling optically hides the fissures and fills cavities that occur naturally in most gemstones, which enables the light to pass through the filled areas making the colour of the stone appear more saturated, and therefore more valuable. It is a multi-step treatment: the low grade ruby is first cleaned with acid to clean the fissures and cavities, and it is then heated up to 900 to 1,400°C to burn the impurities in the fissures. The ruby is then mixed with oxides of silica, lead with sodium and potassium, and is then heated. The result of the treatment is often stunning, as heavily fractured, very low-grade pink-looking material can look like a fine, non-fractured red gem after treatment. Rubies are coloured by chromium, and a small amount of chromium added to the mix can produce fabulous looking stones, giving a colour as fine as Burma ruby. As ever a trained gemmologist will first assess the stone using a loupe and may be able to detect the treatment at this stage by observing tell-tale round air bubbles captured in the glass. Glass filling can by confirmed under a microscope, where the gemmologist can usually see the blue or orange ‘flash effect’ using darkfield illumination, i.e. looking at the stone in transmitted light. This treatment is not permanent as lead glass is not as durable as corundum. The stones used for this treatment have more fissures and cavities than actual corundum, and they can be easily scratched by normal daily activities. The filler can naturally degrade over time exposing the original fissures and factures, and any contact with acids and chemicals can lead to damage of the filler. There is also a more recent tendency for the process to be used on rough rubies, prior to cutting, creating stones that are essentially two pieces of ruby held together only by glass. This treatment was the subject of lengthy discussion at the recent CIBJO Congress in Tel Aviv, where it was felt that in the most heavily filled examples the term ruby was not appropriate and the stones should be described as composites.
Posted on: Tue, 02 Jul 2013 10:37:02 +0000
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