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Gemstone Doublets & GTD's

Today we are going to discuss a traditional emerald simulant, the garnet topped doublet (GTD).


What is a doublet?


A doublet is any gemstone that is made up of two separate pieces glued together. These pieces can be of the same material or different materials entirely. A common doublet that is often seen in antique and modern pieces alike is the opal doublet.


An opal doublet by the Opal Guy

Doublet examples


Opal doublets usually consist of a fine slice of natural opal glued on top of a plastic backing. The goal of the doublet is to give the impression of a much larger (and more expensive) natural stone, whilst saving on cost. There is no issue with doublet stones if you are aware that you are buying one as the price difference between a doublet and the real deal is substantial.

Sapphire and clear quartz doublet- via Etsy

Although opal is certainly not the only gemstone to fall foul of this technique, with doublets being noted in almost every precious gemstone. In the case of sapphires and emeralds the doublets often have a slice of natural stone to the crown and a synthetic pavilion of the same gemstone; the natural gemstone gives the piece natural looking inclusions and confuses many who are trying to identify the gem, whilst the pavilion of the stone makes up the carat weight in lab grown material.


What is a GTD


However, the doublet we are most interested in here is an antique method used to simulate an emerald: the GTD. Here the crown of the stone is a very fine slice of red garnet, whilst the pavilion is bottle green glass. Strangely the optics work in such a way that the red garnet appears bright green when you look down into it, so it can be hard to identify.


An antique ring by SHJB with a central GTD

How to identify a GTD

The 'red rim' effect in a GTD

One tell-tale of a GTD is the ‘red rim effect’, which is essentially where you can see the join of the garnet to the glass pavilion, best viewed on a white paper or through the side of the gemstone. A second is the difference in lustre (the shine of the polished surface) between the man-made glass and the natural garnet. And the third is the absence of inclusions in the base of the stone, replaced by bubbles or swirls of colour where the dye has not been well mixed in the paste. And lastly there may be air bubbles trapped in the glue that holds the two layers together.



This ancient technique is one that isn’t commonly found in jewellery today but one that all antique enthusiasts and gemmologists should be weary of as it can drastically change the cost of a piece.