Possibly a locket, for displaying a braided lock of hair, sometimes as a momento mori. Is the glass part hollow, solid, or 2 pieces? It doesn't appear to be enlarging the finger underneath it, so I'm thinking it isn't a magnifying lens.
The rim around the glass has an opening underneath where the ring is attached. It's possible that the ring attachment will unscrew in order to loosen the rim, and let the glass be removed.
