Anu Manchanda explains the expertise required to check for common treatments in tanzanites and rubies.
Anu Manchanda picks up the tanzanite in her tweezers and holds it under her diamond light. At first sight a non-gemmologist would think it a good quality stone – a deep indigo blue with strong colour intensity; but to Anu’s experienced eye as a gemmologist it just does not look quite right.
Anu moves it slowly under the light, examining it carefully with her 10x loupe. The colour looks unnaturally patchy, particularly when viewed from the crown, and a flash of iridescence as she tilts it under the reflected light makes her put down her loupe and turn to the microscope.
Positioning the stone carefully, Anu examines it under higher magnification, looking closely at the facet edges and culet of the stone. She can see tiny signs of abrasion around the edges, and then, looking at the facets more closely, she finds tell-tale imperfections which confirm her suspicions: the tanzanite has been cobalt treated to improve its appearance.
Anu already knows that if the stone is put under ED X-ray fluorescence it will show relatively high levels of cobalt on the pavilion. The treatment takes the form of a surface cobalt coating, which can improve the apparent colour of the tanzanite by as much as two grades – ie from A or AA to AAA or AAAA. The colour enhancement makes the stone seem better quality, and therefore more valuable, than it was originally.
Treatment of gemstones to enhance their appearance is not a new phenomenon; it has been going on for centuries, with the first mention to be found in a book written in 1548. Common gem treatments include heating, irradiation, bleaching, colourless or coloured impregnation, dying and coating.
The wide and consistent use of some treatments over the years has eliminated the need for disclosure - for example, one of the most commonly accepted practices is the conversion of green aquamarine to blue aquamarine by using heat. For the more drastic treatments, which can mislead customers into thinking they are purchasing a far better quality gemstone than they really are, disclosure is compulsory.
The way that stones should be described and the treatments that should be disclosed are all clearly set out in the CIBJO Blue Book on Coloured Gemstones, which is widely respected as the worldwide ‘bible’ for gemstone nomenclature. The most important task for a gemmologist is to be able to identify the treatments that have been used to modify the colour and clarity of the gemstone under examination.
It is generally accepted that all tanzanites will have been heat treated to transform them from the greyish brown of the rough to the vivid and intense indigo and violet hues that are so sought after. This is an irreversible process and there is no risk that the stone will fade over time, or be less robust as a result of the treatment. However, the application of a surface coating is a very different matter, as it can enhance a relatively inferior stone by as much as two colour grades, greatly inflating its value. The coating is not permanent and will eventually peel away, leaving a very disappointed customer. This treatment should therefore always be disclosed, but this is not always the case, and AnchorCert has seen a number of such stones which have been bought in good faith but have proved to be enhanced.
Lead glass filled rubies
Another common treatment which AnchorCert is seeing increasingly frequently 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 leadglass- 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 be confirmed under a microscope, where the gemmologist can usually see the blue or orange 'flash effect' using darkfield illumination, ie 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.
The forthcoming REACH regulation restricting the use of lead in jewellery, which comes into force throughout the EU on 9 October, will throw another new dimension into this discussion. These ‘composite’ stones will certainly not be compliant with the new legislation unless the glass can be proven to be 'crystal' glass, which has specific properties.
As ever the gemmological world is changing on a daily basis and the AnchorCert team members are working hard to be alert to all the new treatments they may see.
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