Bezel to a hunting case pocket watch...Is my guess. A small watch, probably a ladies watch. The flat spot is the hinge side( they made room for the hinge on the outer cover that snaps down over it,). It may be gold, have it acid tested. Good chance that it is gold. It's hard to tell from pics but it looks pretty good.
They did make a few wrist watches with the cover like that too. But a lot of the small ladies pocket watches were used on necklaces. There was also a maker taking the old smaller cases and making wrist watches out of them....Either way, I'm convinced with no doubt that it's to a watch of some kind with an outer cover like a Hunting case.