Found about 5" down in front yard of daughter's house in Grant Park, Atlanta. 2" diameter, solid brass. When I found it I found one on eBay, exactly like it for $40 and said it was made around 1940 but I have no idea where they got the info or if it's correct.
Do a Google search on "Sam Biern". He made fashion buttons from the 30's to the 60's. Most of his are back marked so I'm guessing yours is a copy or a knock-off.