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 5cm in diameter..