Well, it's a stone that I love, not just for its history but for how it is made...
Garnet Topped Doublets (or GTDs) have a thin slice of red garnet fused to the surface of molten glass – no glue is used. Once it has been cut and polished the join line may be seen in the crown facets of the stone rather than at the girdle as you might spot in other composite stones.
As garnet is harder than glass – 7.5 as opposed to 6 – it makes the table of the stone more resistant to wear and may have natural garnet inclusions giving the appearance of a natural stone on first glance.
First used around 1850, they may be found in antique jewellery imitating emeralds, sapphires or rubies, depending on the colour of the glass used.
Interesting that the red of the garnet does not affect the perceived colour of the stone, for example, the red garnet and blue glass do not look like a purple stone... My favourite thing about this stone is that if you place it, table down, on a piece of white paper you may see a red halo around the girdle – feels a bit like a magic trick.