The words just popped out. Have you tried a magnet on a string, you can test weak magnetic fields that way. Also you didn't say in your post, did you find this with a detector. If not, you could try with one and find out if it is ferrous that way. It does look just like that East Texas iron ore. I used to live in Nacogdoches and the area was full of Austin stone (what it is locally called).