Those are the right regions, but what you need to do are find the caverns that actually lead to underground dungeons, and then find the gem vein nodes in there. So what that means is, on the surface dungeon map, look for "surface with caves." Then go there and look for a passage down into the caves (either a door somewhere in the surface tunnels, or a little ladder thing). Then go into the underground dungeon and explore around until you see some gold nodes. Probably behind a boss node. In those you'll find some gems of various kinds that are likely for the region, and opal will probably be among them.
Good luck!
