If you have nothing else, you can rub the stones against each other in a sort of progression. It does work. Doing this under running water is usually best.

If you have a lot of material to remove then you need something quicker probably - a plate or wet and dry sandpaper on a bit of stout float glass or trued marble/granite is good, as is lapidary grit on glass for very course to very fine removal.

Regards,
Neil