Method of Stone Construction
Most cut opals are solid stones. The entire stone is cut from a single piece of rough (see top illustration). However, some opal rough has very thin but brilliant fire layers. Some artisans cut the stone down to the thin fire layer and glue it to a base of obsidian, potch or basalt - then cut a finished stone. These two-part stones are called "opal doublets" (see center illustration).
To protect the soft opal from abrasion and impact, a crystal clear top of quartz, 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 clear cap, opal layer and base). The photo below shows two opal triplets, one face up and one upside down.