Australian Opal Doublet Jewelry Australian Opal Doublet Gemstones are the slices of opal glued to a black backing, designed to imitate the appearance of a solid opal. The slices of opal are generally adhered to a black backing because the color tends to become much darker and vibrant with its unique placement. However, the costing of the stones is comparatively much lesser than the original ones because of the small amount of genuine opal.