Opal doublets are slices of opal glued to a black backing, designed to imitate the appearance of a solid black opal. Adhering the slice of opal to a black backing causes the color to become much darker and more vibrant.
Doublets can usually be identified by looking at the side of the opal - if the stone has been adhered together you will notice that the line where the colored opal and the black backing meet is perfectly straight. This is necessary for the two layers to be adhered together. If a doublet is set into jewellery with the sides covered, it is extremely difficult, even for an expert, to tell whether it is a doublet or a solid opal. Since the top of the stone consists of pure opal, it therefore appears exactly like a black opal, and thus have a natural appearance.
VERY IMPORTANT CARE INSTRUCTIONS: Because opal doublets consist of layers which are glued together, prolonged exposure to water may cause lifting between the layers and infiltration of water. (This does not mean your opal will be ruined if you wear it in the shower once, or are caught in the rain.) If water penetration occurs, a doublet or triplet will take on a 'foggy' or grey appearance. You may even notice the appearance of condensation inside the stone. You should avoid getting a doublet opal wet to avoid water penetration.
An opal triplet has three layers. Gem opal (usually a very thin slice) is sandwiched between a piece of dark backing material and a cap of transparent quartz. The transparent cap protects the opal layer and adds depth to the triplet. The same care should be given to the opal triplet because of the glued layers.
A solid opal is more valuable than a doublet of similar color, pattern, brilliance and size; and an opal doublet is more valuable than an opal triplet of similar appearance.
(some information shared from LostSeaOpals.com)