Crayfish Gastroliths

I can’t remember when, but a couple of years ago, I think it was a co-op student at work, or maybe a new instructor, come over and ask me what some little thing was that they had found. It was small, flat yet round and slightly depressed on one side, like a danish or a donut whose hole didn’t make it all the way through. I examined it and took a couple of photos, and told them what I thought. “Probably a seed of some kind”, I said, but I never did figure out what fruit that seed came from. Turns out it wasn’t a seed after all but something a little bit stranger.