Heavenly blue green opal doublet set into a sterling silver brooch. 42x15mm. A doublet is a piece of solid, natural transparent or translucent-coloured opal is glued to a dark backing (usually black glass), imitating nature's solid stone. The dark base enhances the Opal's colour. Although some doublets can be quite valuable they are also a very cost-effective way of purchasing a beautiful Opal at less expense.