Doublets are a great way to achieve the color of a gem black opal with a fraction of the cost. They have been enhanced to give the effect of a black opal by attaching or gluing some black opal or black colorless potch on the back of a lighter piece of opal to change its looks.
Even though doublets are opal, they are not natural as they have been enhanced through being composed. So I would consider the opal as treated.
Here at Black Opal Direct we make doublets the old fashioned way with black potch. Bonded to crystal opal the result is a beautiful opal with intense color.