This was said to be a very rare and special Georgian button but see below as it may well be late 19th century. It is of treen (wood) depicting a charioteer who appears to be cupid with horses. This is set within a gilt metal or brass narrow frame with a gilt shank and a plain black back. It is in excellent condition for age and would be a most unusual addition to a collection of buttons. Diameter 1.1 ins (2.7cm). Weight 4 grams.
A knowledgeable button collector has sent me the following information: "This button depicts "Cupid in a Chariot Drawn By Ponies" as seen in the Big Book of Buttons, page 557, number 9. That one is brass. Picture buttons such as this were produced in the latter half of the 19th century, prolific between 1880 to 1900 (ref The Collector's Encyclopedia of buttons p. 150) Yours looks like wood composition and the tin back is most often seen on later 19th century buttons".