The short conical holes/dimples in the thin flat brass disc are to hold rhinestones. At 45mm in diameter, it is too big to be any buitton except a late-1700s "Dandy" flatbutton… so it is most likely a jewelry "pin" missing its attachment device on its back.
Not really.You're probably correct about the rhinestones but it weighs 53 grams which would be quite heavy to wear as a brooch don't you think. And it's about 4 mm thick