Being a rather crudely cast 1-piece solid pewter button from the 1600s/1700s suggests that the emblem on it MIGHT be a thistle flower and leaves, which is the national emblem of Scotland. The Scottish Thistle emblem has been depicted on some buttons (both civilian and military), and Scottish coins (ancient and modern).
As I said, it is such a very crude casting, which is typical of that time-period... that I cannot be sure the emblem is the Scottish thistle flower and leaves, but it looks like a crude representation of it. Compare it with the image below. Of course, I could be wrong.
That's an awesome button! I can see what CBG sees with the resemblance to a thistle..I'm leaning more toward a very early balloon flight.
I also think it is possible that the Civil War button linked above is older than that (but I don't really know). Is there other evidence to link it to the Union Army?