"Assembled" or "Composite" Opals: This illustration shows the differences between: (A) solid stones; (B) opal doublets; and (C) opal triplets. "Doublets" are so named because they consist of two parts. "Triplets" are so named because they consists of three parts.
Method of Stone Construction
Most cut opals are solid stones. The entire stone is cut from a single piece of opal rough (see top illustration).
However, some opal rough has very thin but very brilliant play-of-color layers. To utilize this precious opal material, some artisans cut the stone down to the thin color layer and glue it to a base of obsidian, potch, host rock or basalt - then cut a finished stone. These two-part stones are called "opal doublets" (see center illustration).
To protect the thin precious opal layer from abrasion and impact, a crystal-clear top of quartz, synthetic spinel or other transparent material is sometimes glued onto the opal. This produces a three-part stone, called an "opal triplet" (see in the bottom illustration).
The photos below show examples of opal doublets and opal triplets.