It looks to be a WW 1 Army Coat button. I would just clean it lightly with water & a toothbrush, then soak it in olive oil for a week or so. That will give it a great sheen. Or you can just water it off with the toothbrush & leave it alone. It is all a personal choice. I prefer to olive oil military buttons such as yours, & I have many of them. I have never been sorry with the end result.