How the Opal is formed
The primary formation of the mine is a rhyolite and obsidian flow full of gas pockets. The Opal Solution or silica, was a secondary deposit carried by geyser activity. As a result of several eruptions over a period of time, the opal lays in layers. Most of the layers are thin, resulting in some of the most beautiful triplet opal in the world. Occasionally the layer will be thick enough for cutting a solid opal which is an extra bonus for the finder.
The Opal is Hydrothermally deposited inside hollow geodes in successive layers making Spencer Opal ideal for triplets as the thin layers of extremely high quality precious opal are very transparent with great color intensity. Opal doublets, often used in jewelry, are thin slices of precious opal glued onto a matrix material. Such gems are considerably cheaper than solid opals, yet provide the same play of color. Opal doublets are sometimes coated with a thin layer or dome of clear Quartz to make them more resistant to scratches (opal is a relatively soft gem). These are known as triplets.
To make a triplet the opal is ground perfectly flat parallel to the fire layer until the intense fire shows completely across the surface. Then a flat piece of black Basinite or obsidian is epoxied to the flat fire layer. The opal is again ground flat on the opposite side until the fire layer is exposed and shows its greatest intensity against the black background. At this time the opal should be about the thickness of a hair. A Crystal cap is applied with epoxy and the opal is ground to its final shape. Thus is an Idaho Opal Triplet created.