Doublets are usually two pieces of opal cemented together. They are made by cementing a layer of high-grade opal to a black potch back, which enhances the colour. For economical reasons, many have thin faces. But domed cabochons made from good quality crystal opal, do in many cases, resemble fine quality black opal, which is due to the partial absorption of light by the black back. Their value is only a fraction of that of a solid black gem of the same quality. Although not always encouraged by the trade as an investment stone, the better pieces do bring a reasonable price and are certainly worthy of a place in any good gem collection.