something in the pic for size might help, however it appears to be a general service Eagle button. Civil war is a probability, the backmark would help with the ID. Size would tell where on the uniform the button would go. Very small is a cuff button, larger would be a coat button.
Well, for whatever reason it's now not letting me upload pics. The button is the exact size of a nickel. From what I can make out, the writing on the back possibly says Extra Quality.