I bet it's costume jewelry or a button. Why? Because the back LOOKS as if it's molded and has a piece missing from the center. The spot looks as if something weas glued on and fell off. Pin back or button shank. Best guess: coat button.
How to test? Tab the black stone are against your teeth. Plastic gives a dull thunk. glass or stone gives a sharp tap. If it's plastic, then the metal is NOT silver. If it's glass or stone, then it MIGHT be silver.
Sage(tap!)Grouse