John in New Mexico
>though I won't go so far as to speculate how they affect sharpening. First, Wilbur is correct that natural stones are sedimentary in nature, and almost certainly grainular in texture, rather than a precipitate like limestone (too soft to sharpen steel). The mistaken assumption in my opinion is that these stones would be as uniform in compostion as a man made abrasive stone. I would guess that the variability of minerals in natural stones is of more relevance than the uniformity or lack of it in particle size. The other variable, which is actually related, is that natural stones of this kind would likely be more friable than a man made one, also due to its heterogenous mineral composition. This is a bit analogous to diamond crushing less easily than SiC, and thus behaving differently as an abrasive material. I would suspect that the friability of the stone is what produces the matte effect, though I couldn't tell you why it is also sharper. Perhaps the availability of fresh abrasive cuts more efficiently and dubs the edge less.

John