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Side-effects of Glass-filled sapphires

A new method of enhancing sapphire is emerging that makes great-looking sapphire more affordable, but jewellers should be aware of side-effects. FGAA’s Megan Austin discusses.

Adding to our series of gemstone treatments is the emergence of a new, improved generation of cobalt-doped glass-filled sapphires.

Much like the glass-filled ruby, this sapphire treatment uses low-quality base material that would be deemed unfit for sale in its untreated form.

Glass-filled rubies rely on introducing yellow-coloured lead glass into surface-reaching fractures to give the appearance of a fine quality ruby.

In the case of near-colourless sapphires, similar fractures are injected with cobalt-doped lead glass to enhance clarity, while bestowing a rich and vibrant electric blue colour.

This latest generation of glass-filled or “composite” sapphires have various features in common with glass-filled rubies that are easily identified with a 10x loupe, including bubbles in the filler and a flash effect.

A simple screening test can easily distinguish these stones from natural, heat-treated, titanium-diffused or synthetic material. This preliminary identification can then be confirmed using standard gemmological equipment.

Developed by Thai gem treater Tanusorn Lethaisong, this treatment was originally named “super diffusion Tanusorn”, although it did not involve diffusion but rather an infilling of cobalt-coloured glass into highly-fractured corundum. This resulted in low-quality semi-translucent-to-opaque material with a deep, dark-blue colour.

A refinement of the technique has resulted in a significant increase in transparency and a decrease in blue saturation.

Christopher P. Smith of AGL (American Gemmological Laboratory) describes these treated stones in his Prestige Gemstone Reports as “composite sapphire”; however, World Jewellery Confederation CIBJO advises that they may also be described as ”cobalt-doped glass-filled sapphire” or “glass-filled sapphire”, but never simply ”sapphire”.

A report published by The Australian Gemmologist (Jan-March 2013) regarding the stability of these treated sapphires warns that any silica-based glass filling such as this is inclined to have durability issues because of its ability to be etched and dissolved by acids. It is recommended that exposure to acids, heat or ultrasonic cleaning be avoided.

Jewellers are advised to remove these treated sapphires from their settings before any repair work is carried out to avoid the risk of damage.

As with many treatments, members of the trade dealing in these goods are expected to have information readily available for the customer, including the type of modification the gemstone has received, as well as any special care requirements.

As always, jewellers must be aware of the need for disclosure. Under the ACCC Act, “Businesses which fail to disclose gemstone treatments, where the value of treated gemstones is significantly less than the value of an equivalent untreated gemstone, may risk contravening the misleading and deceptive conduct provisions of the Act.”

Hence, it is important for jewellers, retailers, gemmologists and valuers alike to be familiar with this material and the methods of identification.

Like all new kids on the block, cobalt-doped glass-filled sapphires may divide opinions in the jewellery trade; however, there is no doubt they present an attractive and affordable alternative to the untreated equivalent.

Megan Austin

Megan Austin FGAA FGA Dip DT BA, is a gemmologist and registered valuer. She operates Megan Austin Valuations.

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Tuesday, 18 February, 2020 09:15am
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