This could well be just a glass. It however could also be an opal
doublet or triplet.
A doublet consists of a layer of opal with a glass (or other
material) layer glued under that.
A triplet has a layer of opal between 2 layers of glass (or another
A good way to show this is to place the stone (with tweezers) in a
glass of water and viewing it from the side, that should reveal the
different layers to you.
It goes without saying that a doublet or opal triplet has little
Alain van Acker FGA