VERY nice button. I think it's coat and not cuff. Don't know rarity or value, but I've been hunting for many years and I have only found a few. If you can see gold gilt, drop it into a a little vinegar, let it soak for about 5 minutes and lightly brush it with a toothbrush to brighten the gold that's left, then rinse in water. If you can't see any gold, leave it like it is.
Great button. Not sure where you are digging but it could be Civil War or Indian Wars. either way, great find. I find a lot of general service buttons but I've only found a few C Cavalry buttons and even fewer I Infantry buttons. Nice save. Thanks for posting.