Here is one that has me stumped. It was purchased as a possible coprolite. However, unless the critter ate a lot of dirt, I'm thinking it is some sort of steinkern. It has a smooth texture on the outside (like it was coating in a thin layer of iron-rich silt, but is very gritty on the inside. It was found near turtle remains. I will post a microscopic image in the next window.
I have come across similar configurations (on a much smaller scale) in the Rattlesnake Creek micro matrix, but I don't know what those are either. I apologize in advance for the quality of this photo. I would happily retake it at a better resolution if anyone thinks it would help.
Thanks for looking!