These imitations of more expensive, transparent gemstones such as amethyst and peridot were widely used in the last half of the 19th century. They consist of a very thin slice of garnet fused to the top of a cut-gem-shaped glass blank. Garnet fuses to glass easily, and is harder than glass, so the 'stone' was less prone to scratch. The red garnet slice is so thin that the visible color comes from the glass.
Manufacture of doublets was often a family affair, with the members sitting around the kitchen table to use moulds to assemble and fire the garnet slices and glass to make 'gems'.
(See the blog for item #1416939 for a more complete description.)