There is one type of button it could be -- but I'm not saying it's a button until it looks like one after proper cleaning. "Four-hole" buttons, commonly used as longjohn underwear-buttons and pants-closure-and-fly buttons, were made of thin stamped sheet-iron, brass, zinc, and more recently, aluminum. Iron and brass ones were made sometime before the civil war, and onward right up to today, so there's no way to accurately time-date one unless it has a maker's-marking on it. Again, I'm not saying thedukeofdelaware's rusty thin disc is a 4-hole button, just that iron ones do exist. They are a common find in civil war army campsites... and Span-Am War sites, WW1 sites, etc.