I was also taught that about stoneware. It really depends on how you are defining clay.
I have seen articles about defining clay by their water absorption. Cant find the exact one but here are a few links talking about similar things.
If you scroll up a little on the google books link it talks through what a clay body is and such and such. Couldn't get it to link to the right page... but go from page 139. It is worth a read as it goes on to talk about lots of different clays.