I recommend keeping two stones in your kit. One with a medium grit (around 800 or so) to perform major sharpening jobs, and one with a fine grit (at least 2,000) to tune the edge to a razor-sharp finish. For real pros, a stone with an ultra-fine grit (8,000 and above) will leave a mirror-like finish on your blade, but most cooks won't notice the difference in terms of cutting ability.
Four hundred million years ago in a small geographical region of Central Arkansas, now part of the Ouachita Mountains, nature began the creation of a sedimentary rock formation composed of microcrystalline quartz called Novaculite. The word Novaculite comes from the Latin word novacula, meaning razor stone. Novaculite is a recrystallized variety of chert. It is the stone material from which whetstone and oilstone sharpeners are made.