Is this an opal?

[realitycheck-7]

I think this may be an opal, the back looks like a sort of rock/geode outside.  I've never seen one like this.  Any help would be great.

[realitycheck-7]

[zienty]

Maybe Labradorite?

[realitycheck-7]

No, it's not labradorite.  I've had a lot of that and this is very different.

[old.crows.nest]

The setting looks like it should hold a Mexican Mask stone. What do the Hallmarks look like?

Edit: If this is from Mexico my guess would be Mexican Boulder Opal.

[treasuresformeandyou]

I'm not great at ID'ing opals, I've found this site very helpful:

http://geology.com/gemstones/opal/

from a quick look at that site, I would agree that it looks like a boulder opal, with kind of a rough, unpolished looking surface.

[realitycheck-7]

It's only marked 925 no other signature.