It's only about 1.5" in diameter and probably brass (it's heavy). I can see two spots on the back where it looks like it had posts or a bar for fastening something. Is this a belt buckle?
Your find is a solid-cast brass US Army horseharness "Great Seal" rosette, dating from the very early 1900s through the end of the widespread use of horses in the army, in the late-1930s. However, this type of rosette is still in service today when the army uses horses for Ceremonial purposes (such as parades and memorial ceremonies). Yours originally has a solid cast brass staple-shaped bar on its back, for attachment to the leather harness straps. See photos below.