Opal doublets are a term for a sliver of natural opal which has been glued to a dark backing, to highlight the colour plays. Many different types of backing are used, including black plastic. These particular opal doublets are of a better than average quality and have been backed with natural ironstone from Queensland, Australia.
As previously mentioned, there is the possibility that water could get between the layers and cause the stone to go cloudy, which is irreversible.
However, if treated with care, they are a pretty form of opal which can be purchased at a fraction of the cost of a natural untreated opal.