A garnet and glass doublet, also known as garnet-topped doublet (GTD) consists of a thin cap of almandine garnet fused to a piece of colored glass imitating a gem. The doublet was popular in the 18th and 19th century, and may still turn up in antique jewelry today.
The reason for constructing this type of doublet is to increase the surface luster on the table of a gemstone. For those who are unaware may think the stone is valuable, but it is easily detectable by the following means.
- The first step is to clean and examine the stone. Visual observation provides you with the chance to examine and classify all aspects of the physical appearance of the gem.
- Check the refractive index. Refractive index of the table will be that of almandine (1.76 or above).
- Refractive index of any pavilion facet will be within the glass range (1.50 - 1.70).
- Use the microscope. The luster difference between garnet top and the glass will become obvious.
- Do the red-ring test by placing the garnet-topped doublet face down on a white background. You will see a faint red ring inside the girdle outline representing the boundary of the garnet cap.
- Immersion technique. When you immerse the stone in a suitable liquid, you are able to see any color difference between the two materials.
- Fluorescence. This is a complimentary test. View the gem parallel to the girdle plane under long wave and short wave ultraviolet light. Garnet will show inert reaction, while glass may show chalky fluorescence.
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