Update post cleaning. Looks like a small locket maybe… there is a bit of greenish tarnish inside but no pictures. I can maybe see a tiny point at the top where a very small loop could have been connected.
This looks like it could be a button, but there is no shank or anywhere it looks like it had a way to be attached (back is smooth.) The engraved inner circle is raised from the wider circle around it. It looks like it's about the same size, or slightly smaller than a dime. Any other ideas what...