IDK! When my dog ate a dime they had to do surgery to remove it. Did he swallow the whole thing? Maybe ask this on the community board, more people will see it! Is there any way to call your vet?
I think you'll be seeing a more or less intact rubber band in the litter box shortly . My three ds use covered rubber bands for ponytails all the time (the kind with a little metal band). For some reason the cat goes nuts for these and gobbles them up whenever she finds them - then there they are in the litter box. She must have done this 20 times (yes, she is an elite-level weird kitty). So now the kids are a lot more careful about leaving the bands where the cat can't find them.
and it did come out. I think it did need a little help though. My dog has also eaten things like thread and string. It just takes a little help getting out sometimes. Make sure it's GENTLE help!!!
Call your vet. If it's small, it probably won't be a problem. But if it's a big one, and especially if it was broken and therefor longer, you may have a problem. I sew, and a woman in a sewing email group I'm a member of lost her cat because the cat ate some thread and it got tangled up in her stomach.