I'd say it's at least 19th century its a brass flat button that's missing the shank.
That rope border I've seen on regiment buttons dating back to the Revolutionary War.
Can you make out possibly a number in the center of the front of that button?
Or any writing it all on the back? A little warm soapy water might bring it out a little
It might just be my eyes playing tricks but it seems like I see an eagle with his head turn to the left.
If its not a civilian button it could be a Union Navy button from the Civil War era