Long time no see.
After things here in my brain settled down, things down lower in my abdomen decided to go a bit crazy, but after a couple of doctor's trips, some anti-biotics and a few days' bedrest, I'm back to my former rocky ways.
This is a razor-sized Maruka Nakayama hone, from Hatanaka hones in Kyoto. It's an off-white color with extensive black specks and a touch of Yake? in the corner.
I have honed about a dozen razors on this one so far, using slurry, and the only thing I can say is: Oh baby.
It's gorgeous. Silky smooth and fast, fast fast. It eats steel like cookie monster going through a box of Thin Mints (mmmmm...Thin Mints.) It even seems to speed up the slurry from my Nagura?--meaning the slurry progression actually works faster than on other stones. Within just a few strokes, the slurry started to darken deeply. Even on water alone, the stone started to pull off black steel visibly.
It's a hard stone, too, slow to take on water and slow to give up any slurry, even when honing Kamisori with pressure.
This means it's easier to control the balance of new/old particles, letting you control the edge much better.
And oh, the edge...just joyous. Best stone I've touched, I am confident of saying.
Now, here comes the rant (you knew it was coming, right?).
You'll notice that in this discussion of a Japanese water stone, I did not use the word "fine." As in "This stone is roughly as fine as a 20,000-30,000 stone."
That's because, in discussion of any natural stone, "fine" is meaningless. Not just because of the old "natural stone particles are irregular" argument, though this is true, but because they aren't.
According to a study done by Tokyo University in 1982, as quoted in "The Charm of Kyoto's Natural Hones," the main abrasive in Kyoto finishing hones are particles of Silica, ranging from 2-3 microns in size. 2-3. That's the size of the particles in a 4K to 6K artificial stone. ALL of the stones they measured fell between those marks, from a variety of mountains and strata.
Of course, I am not arguing that these stones are 4-6K, not at all. They are finishing hones, they leave a smooth, sharp edge perfect for shaving.
The reason they can do this is NOT because of the fineness of the particles, but because of the shape. Again, the Tokyo study can help explain this. The shape of the particles, rounded flakes of SiO2. They don't gouge the steal, they take out little divots--especially in using slurry. So the amount of steel being removed is not only small, but also done in such a way that there is no saw-toothed edge like that left by artificial stones.
Of course, they cry, "the reason we compare these stones to 20,000-30,000 grit hones is because the edge they leave is comparable!" To which I say "What does a 20,000 grit edge feel like?" As far as I know, there is only one company in the world that makes a 20,000 grit hone, and one company that makes a 30,000 grit hone. The particle size of a 30,000 hone's particles are .49 microns, a 20,000 (theoretically) is .74 microns, and a 15,000 is .98 microns--the range there is so tiny, I find it hard to believe that the human face is really capable of telling the difference between those. Really.
But to be honest, the important point is this: the difference between Japanese hones is NOT particle size. You simply can't compare stones like that, because they all have similar BIG particles. The differences you see between them are based on the hardness of the stones...
It's accepted wisdom (I haven't seen a lot of real scientific evidence, but it's universal enough, among people with decades of experience, that I'll give this the benefit of the doubt) that the particles on Japanese stone slurry break down in the process of honing. As they get smaller, they get finer and the edge gets smoother and better. However, if you have a softer stone then it is constantly releasing new particles and you never reach that final stage of edge refinement. Thus, you are getting an edge off those big old particles, and the stone will feel coarse. A hard stone, then, releases particles much slower, and usually only when needed--so it finishes smoother and "finer".
Among Japanese finishing hones from Kyoto, which all have similar compositions within small tolerances, "fineness" comes from hardness and from proper use. ALL awaseto, the Kyoto finishing stones, then, are equally fine--or equally coarse.
So the key factor in choosing a Japanese Finishing hone is not fineness. It's hardness, qualified of course by the purity of the stone (other inclusions can effect speed, "scratchiness", etc. of the hone). A great hone will have the right level of hardness for your blade, a good rate of speed at taking off steel due to the balance of other elements of the stone, no hard "suji?" (toxic inclusions that can damage your blade) and an appealing figuring.
Fineness is something that will happen in the hand, not in the stone.