Japanese natural sharpening stones are becoming scarce because many of the mines are exhausted after centuries of quarrying them. These stones are mined in the Narutaki District near Kyoto and are well regarded by professionals. Japanese stones are softer than stones found in other areas and because of this they do not glaze as much as harder stones, since new material is constantly being exposed as you sharpen.
There are basically 3 categories of stones. Ara-to, which is a more coarse stone (500-1,000 grit); Naka-to, which is a medium grit stone (3,000-5,000 grit); and Shiage-to, which is a fine grit stone (7,000-10,000).