13mm, Pre Civil War era, New York Militia cuff or kepi button. The backing is made of tin and has rusted out in the ground.   This is quite an unusual button. It has a very similar face design to NY12 through NY-15; however, it is of two-piece construction and the size and tin back would indicate it served as a side button on a hat/cap.   It will make a nice addition to someone's collection.