A Doublet Opal is a layer of genuine Natural Crystal Opal with a layer of Australian Boulder Opal "potch" (or ironstone). The two layers have adhered with silica-based adherents that have the same coefficient of expansion as the Opal so that the layers do not delaminate over time. Opal Doublets are guaranteed for life against delamination. Doublet Opal Earrings are rare and valuable as the matching colors are almost impossible to replicate in the more expensive solid Opal.