Triplet Opal:A triplet opal is a type of opal that has three layers. The top layer is a thin layer of clear quartz, the middle layer is the opal itself, and the bottom layer is a dark backing, usually made of black plastic or obsidian. Triplet opals are much cheaper than solid opal, as they use less of the precious opal stone. However, they are also more fragile and can easily crack or chip if not handled carefully.