Triplets consist of three layers:
A black backing as above A paper-thin slice of opal in the middle A clear glass, quartz, or plastic capping in the shape of a dome. The slice of opal in a triplet is usually extremely thin (paper-thin) so the clear capping serves to give the stone a nice cabochon on top. The clear capping may also magnify the colour of the opal slightly, and also serve to protect the opal.
Triplet opals are natural opals that have been stabilized to prevent cracking and improve durability. Triplets are also man-made. The majority of triplets are made from black potch, which is made from a mixture of ironstone and quartz. The top layer is clear non-opal material that has been fused to the black potch layer. However, the backing can be made of other materials such as black potch or brown ironstone.
Triplet opal is just as beautiful and valuable as doublet opal. The only difference is that the top of the stone is made from a synthetic material. Triplets contain less real opal than doublets. The top of the stone can be made from synthetic material. This is why triplets can be a lot more resistant to impact than solid or doublet opal stones. (Opal is a fragile gemstone.)