Thread: What actually is Hardness in natural stones, and how it affects sharpening

I was just reading this thread again, very interesting points.

I only have 4 natural stones. A takashima karasu, an okuda suita, a nakayama
namito(I guess,got it from mainaman) and a chinese guangxi hone. When I tap
all stones:

-the okudo doesn't really give a high pitched voice, the others do.
The okudo also feels softer and produces easier slurry.

-The karasu, namito and guangxi are close in hardness. All three give high pitched
sound. They give their slurry very slowly.

Then I tried scratching one stone with another.

1)The karasu did not really create any scratch on the chinese guangxi hone(CGH) and
vice versa. My conclusion, both stones are about the same hardness.

2)Used the namito to scratch the karasu and it did. The karasu could not
scratch the namito. The namito also scratched the CGH.

Conclusion, the namito is the hardest. This is hardness based on the scratch
test, feel of the stones and the sound they give when tapping the stones.
A clear caveat is of course that the sound given after tapping has to do with
the size of the stone.

The scratch test is confusing for me because I would guess the sharpening particles
in all japanese stones have the same hardness. It's just the how tough the binder
is and how much compression there is that AFAIK defines hardness in these stones.

Sharpman