Hi; It's a WW1 Army Coat button ok. Cleaning, I soak it in Lemon juice for a few day. Then use a tooth brush on it and repeat until clean ok. Greetings from Boston. Welcome to TNet. Look forward to your posts. PEACE:RONB
Subterranean's guess (he said "looks like") is correct. Your button is an 1875-1901 US Army "general service" button for use on Enlisted-men's uniforms. It was replaced in 1902 by the "Great Seal" general service button.
Your button is very similar to the 1854-1874 version, on which the shield wasn't as highly raised. Compare the three versions of US Army general service buttons in the photo below. Great Seal at left, 1875-1902 at center, 1854-1874 at right.