(term used throughout the opal industry). A layer of precious opal, onto one face of which has been fixed an opaque base of natural or artificial material. The base is generally black to enhance the opal's fire; it may be black potch, or more commonly a black plastic. Mintabie "semi-black" opal is a natural doublet.
An opal doublet is a manufactured stone that is composed of two thin layers that are glued together. A thin layer of opal is glued on top of another mineral (usually a black onyx or ironstone, which enhances the stone's color), producing a stone that is less expensive than a solid opal. Doublets must be cleaned very carefully