Step 2: Advanced Gemology

Lesson 11


Assembled Stones

It is common to glue two or three pieces of material together to make a single stone. Opals are frequently formed as doublets, a thin layer of opal on a secure backing, or triplets, with a clear quartz cover. This is done to make use of thin, delicate material and the layers are visible without magnification.

Assembled, faceted stones are more difficult to distinguish. Inexpensive stones use colored glue between two layers of colorless, synthetic material. Others are designed to deceive the gemologist. They have a top of natural gem material on a synthetic bottom, or a natural but colorless bottom with dyed glue.

Doublets are occasionally visible from the side, however, it often it takes immersion to distinguish the layers. (This technique is explained in “Using a Microscope”.) One of your best clues is finding bubbles in the glue layer.


You have to be very careful when inspecting your stones. This doublet is well cut, something you wouldn’t expect from a $1 gem. It also just has two small bubbles in the glue layer. These are indistinguishable, so it would take further testing to determine the…