Step 2: Advanced Gemology
Two or three pieces of material can be glued together to make a single assembled stone. Very frequently, treaters form opal cabochons with a thin layer of opal on a secure backing (a doublet), sometimes with an additional clear quartz cover (a triplet). These techniques make use of thin, delicate material, and you can see the layers without magnification.
On the other hand, distinguishing assembled faceted stones can prove more difficult. Inexpensive stones can have colored glue between two layers of colorless, synthetic material. Others are designed to deceive gemologists. They may have tops of natural gem material on a synthetic bottom or a natural but colorless bottom with dyed glue. (Center-fused color treated gems use an HPHT process instead of glue to fuse diffusion-treated pieces).
You can occasionally distinguish doublets by viewing them from the side. However, more often, it takes immersion to distinguish the layers. (Use the immersion procedures for a microscope examination). One of the best indications you’re looking at an assembled stone is finding bubbles trapped in the glue layer. (In the pavilion view of the spinel doublet above, those two indistinguishable…