Awesome. I've been researching them. I found that they are argillite, which I believe is another name for slate. There is a wonderful "rocks of North Carolina" website. Maybe those who are trying to identify their own rocks can look for a rocks of "state".
Thanks again. I think they will do well in my tank.
Just in case - if they are soft, I would soak one in a bucket for a few days and make sure it doesn't disintegrate. I had some nice fossils in mudstones that were soaked for me and completely fell apart.