Also known as "composite opal" or "assembled opal".
"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.
Opal Triplet: This photo shows two opal triplets. The one on the right is in a face-up position and displays a bright play-of-color. The one on the left clearly shows the thin piece of black obsidian that serves as the base. Opal triplets consist of three parts: 1) a thin base (often black obsidian, plastic, basalt or glass); 2) a thin slice of precious opal; and, 3) a clear top of quartz, synthetic spinel, or other material that protects the opal and sometimes serves as a magnifying lens to enhance appearance. Opal triplets are more durable than opal doublets because the fragile layer of precious opal is protected by the cap. They are also more durable than solid opal.
Opal Doublet: This photo shows a face-up view and a side view of an opal doublet. In the face-up view it looks like a gem cut from solid opal. However, in the side view you can see that it consists of a piece of host rock on the bottom with a thin slice of precious opal at the top. A thin glue line can be seen in the side view. If this opal was mounted in a cup setting, it might be impossible to tell that it is a doublet rather than a solid stone. Opal doublets sell for a small fraction of the cost of a solid stone with similar face-up appearance. Opal doublets are less durable than opal triplets because the fragile opal is exposed to impact and abrasion. They are best used in pieces of jewelry, such as earrings, that are not exposed to rough use.