Before the availability of synthetic materials, knives in Japan were sharpened on natural sharpening stones that were hand quarried, hand cut and hand polished. Although some high level craftsmen and woodworkers still insist on these extremely expensive natural stones, modern technology affords today's knife sharpeners a wide variety of highly effective synthetic, ceramic and diamond surface stones.
Along with determining which grit stones you will need to match your knife and your sharpening skill level, you should also compare stone characteristics to make the right stone choice for you.
For many users, sharpening will begin on a medium grit stone and progress to a fine grit stone to finish the edge.