I am no button expert, I'll try. Looks like the front piece of a two piece stamped U.S. military uniform button. The shield signifies Army, you can make out 3 arrows clutched in the eagles left talon. You can clearly see an "I" and "S" at the top, It would be possibly Infantry. The bottom I can see M I C and a letter that maybe you can make out?
The eagle is facing to the left making it close to the Civil War years, around 160 years old. There are books with extensive knowledge on American Military uniforms, buttons and Insignia which I have none. I do not think they are worth much although a piece of American history and I would be thrilled to find that.