Opal is prized for its beautiful inner display of color caused by light diffraction. It is often used as a cabochon to best present this characteristic. Every opal will have a different play of light. When purchasing opal, it helps to be aware of a few terms. Solid opal is just as it sounds (pure opal). An opal doublet consists of a thin layer of opal with a dark background behind it, while an opal triplet has three layers: a dark background, precious opal, and a clear protective top layer.