Natural stone are not pure, and grits are even, but many people collect them because it's natural, lol. For better sharpening, just use a bloody man-made stone, man!
It's a matter of natural particle friability. Synthetic carbides are not friable they are too hard to break down. In other words these particles slowly break down to finer and finer grit in the stone slurry as the blade is passed over the working stone. Creating finer and finer scratches with each pass as its removing course scratches.
