|An opal "triplet" is created when a thin layers of precious opal is placed between a dark backing and a clear quartz top. See the second image. The back material is often "opal potch", opal without any fire and usually quite dark or gray. Look closely and you can see the "fire" opal just above the black backing.
Above the fire layers is a piece of clear quartz. It protects the much softer opal, and acts as a mini-magnifying glass enhancing the colors. This particular triplet was made up of a single piece of precious opal.
This is a good way to use very thin seams of precious opal that could not be cut into stand alone stones.