I've cracked a few of these smaller ones for funsies and they just have either nothing or these little brown crystals near the center. I honestly forget what mineral even though someone told me but I remember thinking it looked like barite? They're pretty small and I didn't have my hand lens with me.

There's a YouTube channel that does videos where all the guy does is crack open concretions found on the English coast. Let me see if I can go find it. Ok apparently there are a bunch, so here's the first one I clicked on: https://m.youtube.com/@YorkshireFossils

These are the same concept as the concretions above but different scale and age.

I'm not sure about the source deposit for the original post photo but I've seen concretions like those in the photo in glacial clays as young as 15,000 years old.

I've also collected little shrimpies in concretions found in Eocene shale (like 40 million years old) and then of course the YouTube channel digs up these crazy good ammonites and other things from the Jurassic that are 175-180 ish million years old.

It's pretty neat. Sometimes they form from the interaction of infiltrating water and bacteria, and there are other types of concretions too. They aren't always calcium carbonate.

Plus, rarely, they can get pyritized and I forget the exact process but it is a form of mineral replacement after (?) the concretion formed (?). I've always wanted to find one that has turned to pyrite but I think they oxidize and tarnish pretty quickly.

Not a paleontologist, just a regular geologist, so that's about all I know on the subject haha.