This is a glass cabochon.
This occurred to me, too-- looks more like dichroic glass than opal.
If you look at it with a microscope or a strong loup, if it is
glass, you should see bubbles. They will be either round (a dead
giveaway) or elongated.
If you have a copy of the January issue of Art Jewelry Magazine, it
contains photos of inclusions in opal simulants, taken by our own
James S. Duncan, G.G.