Your button is from the uniform of a Virginia Military Institute cadet. That specific type was manufactured before and (long) after the civil war. (VMI cadets still wear that emblem today.)
To date your button accurately, we'll need to see a well-focused closeup photo of its back, showing the exact wording and punctuation of the maker's-mark (backmark) on it.
The most common pre-civil-war backmark on VMI buttons is R&W Robinson. A "D. Evans & Co." backmark can be either pre-civil-war or postwar, depending on whether or not the "D. Evans & Co. is written inside a ribbon.