Is the setting open in the back?
I am doubtful that it is natural opal...mainly because who puts that nice of an opal in a sterling setting? If the setting is open in back it may be easier to tell what it is than looking from the front. IF a manufacturer did put that nice of an opal in sterling, they may well have left the setting open in the back to demonstrate that.
I agree with @caldreamer that the play of color looks more foil like, the way you'd expect a simulated opal to look. But photos can be deceiving.
Although the photos look suspicious, a likely possibility is opal triplet. This is a very thin slice of natural opal cemented on to a black onyx back, with a clear quartz top. Triplets often look like fine black opals. You would want to look at the edge (conveniently covered by the bezel setting) and the back (possibly also conveniently covered by the setting).
On the edge, a black opal would follow the natural lines of the opal top to the matrix back, whereas an opal triplet would show a very straight line of assembly. Looking from the back, a black opal would have a dark natural rock matrix, whereas a triplet would be smooth and black like an onyx.
I'm not sure how expensive is expensive to you, but a natural black opal that nice is thousands of dollars. A nice silver piece with an opal triplet is likely a modest few hundred bucks. But opal triplet would explain it being not inexpensive and from a reputable jeweler.
