The button is 18th century, probably 1770 to 1790 and is painted on vellum or ivory,
with reverse painting on the glass which makes it look to be 3-D.
The brown streaks are damage where water has got into the button.
It is held in brass.
It would have one of a set that was worn on a man's coat.
In the last quarter of the 18th century, men's fashions became very ornate and buttons became larger, up to 5cms in diameter.