wilbur
>The paradox of uniform vs. random particle sizes in a natural sharpening stone and its performance can be easily resolved. It is clear that man made stones can be made more uniform than natural stones. However, natural stones have random particle size within a fairly narrow range.

The layers of rock that provide natural sharpening stones probably are sedimentary in nature, judging from their characteristics. These layers were probably deposited by natural currents of some sort. The fact that these sharpening stones come from specific layers indicates that the particle size is relatively uniform.

Think of the last time you were walking on a beach as the tide pulls out. If you look closely at the waterline, you'll see that the grains of sand have some organization by size. This is because sand grains of different sizes will be carried different distances by the same current of water. Multiply this process over the time needed to create a layer of sedimentary stone, and you'll wind up with areas of relatively uniform particle size. Pick the right area, and you'll have a sharpening stone mine.

In Toshio Odate's book on Japanese tools, there are some micrographs of sharpening stones and blades sharpened by them that clearly illustrates this point.

It may be that the current crop of man made stones have particles so uniform in size that the performance of these stones will differ from natural stones of the same nominal grit size. Whether this translates into improved sharpening performance is actually a different question. Remember, the metal particles that make up the blade are also going to have some randomness in size as well.