Ohiochaplain, although the buttons in your two photos are both 1-piece buttons, they are not the same type. The button in your second photo is called a "spun-back" 1-piece. That type was cast in a mold, and its back then shaped with a lathe. According to the Tice book on buttons, this type was manufactured from the 1770s to about 1800. Their metal, which usually has a very smooth grey or silvery appearance even after 200 years in the ground, was a composite alloy such as Britannia Metal (93% tin, 5 % antimony, 2% copper), Tombac (85% copper, 15% zinc), or White Tombac (80% copper with some zinc and arsenic). Britannia Metal was silvery, and extraordinarily resistant to corrosion, but brittle. The rust you see on your button did not come from the button itself. The rust on it is a stain caused by the button having laid against an iron object in the ground, or kept (after digging) in a container where it was exposed to rusty water. If I owned it, I would use some rust-remover to get rid of the stain.
Regards,
Pete