Opal Doublets. Boulder and matrix opals have sometimes been referred to as “natural doublets” although, in my view, this term is an oxymoron. These natural materials are fashioned with their thin opal seams showing on the surface of the natural matrix in which they formed and are not assembled stones. Boulder Opals –not assembled stones. Opal triplets have a colorless, usually somewhat domed, cap cemented to the doublet.