An "opal-triplet" is made up of three sections, a (top) polished quartz cabochon to protect the opal layer, a center opal slice , and a bottom piece of "potch" or dark material to provide strong contrast for the center slice of opal, and protect the rear of the stone. The contrast of black as seen through a thin slice of opal makes it look like the highly prized "black-opal". An "opal-doublet" is similar to a triplet, but has only the polished quartz top and the slice of opal. It does not have the backing material. As a result the slice is usually thicker, and the stone takes on more of the characteristic background color of the original opal.