Barker is correct... more specifically, it is an early-war (1861-62) version of Confederate Infantry uniform button, made of solid cast pewter. Most civil war military button collectors call that version simply a "Pewter I" button. Due to scarcity of pewter in the wartime South, none of these seem to have been manufactured after early 1862. It's a good bit scarcer than brass 2-piece Confederate Infantry buttons.
Remember, where there's one of these, there's usually more nearby... along with other Confederate relics.