That's not briar at all! That's meerschaum, and probably African block at that. It looks to be fairly old as well. I'm not sure that the stem is original to the pipe, but it'd probably be worth fixing up.
Sorry I said briar in my post but meant meerschaum
The stem is over rotated when tight and so is loose when straight . I don't know what it should be if this is not the proper stem. I was thinking of making a spacer to take up the slop for now.
I think this is a Tanzania pipe.