Looks like it could be a winner. Probably someone sharpened knives on it and it can be saved. Get yourself some Wet/Dry 80 or 100 grit sandpaper, draw lines with a pencil on the stone and with water on the paper flatten the stone. Keep redrawing the lines so you are really sure it is flat. Next get 220 paper do the same and progress up into 1200 paper of something close. Before you try a razor on it be sure to use a stiff brush to scrub off any of those loose grit particles from the sandpaper. Of course you have to start with perfectly set bevels on your razor before getting any good edges off this Jnat, but the stone should be able to do the final work.

Your stone looks like an asagi, lets see some photos of the back.

good luck,
Alex