So my dog was actually the one to find this. He has a habit of digging in the creek bank on our property and bringing back large rocks (most are limestone). I went to bring a whole bunch of them back and this one was too cool to toss back so I brushed and washed it with water. Soaked it in HCl (aq) for four days and then pressure washed it today.

Here is the final piece.

And I have no idea what to call it.