I've had a couple people ask if I knew what kind of rock in my last post was. I believe it is a hematite aggregate. I found a larger version which I now post.
It is massively larger than the last rock.
I did a little cleaning of the smaller stone with muriatic acid. It got rid of the calcite between the nodules
It also exposed some white opal between the nodules
So remember: even ugly rocks can be cool!