Somebody mentioned oilstones. I have a couple natural oilstones. I don't know that there's anything necessarily mysterious about using them, per se. I used both sides of the stones and attempt to keep the wear even. Mind the corners and the middle will take care of itself one writer has written. I think that's true.
The Washita has a 'good' side and a 'bad' side and I'm reasonably sure old timers would have sunk this stone in plaster, bad side down of course. I use the bad side often - it's a little grabbier but faster too.