For traditional finishing, it would be a tomonagura--a piece of the same stone used as a slurry stone. True nagura are much coarser than the actual stone & are used for preparing the edge for the final tomonagura slurry, which can be raised w/ a diamond plate if you don't have a matching slurry stone. For finishing, you use slurry from the main stone or water or some combination of both, depending on the stone. FWIW, I use a little folding DMT-C minisharp on mine--same size as a slurry stone even. :)