Quote Originally Posted by DaveW View Post
I think the nagura is pulling all particles from the surface of the stone and not just iron oxide particles. I don't know what the composition of the stone is itself, but most stones that have a significant amount of iron oxide are reddish or rust colored (you probably already know that).

If you get a clear bright finish with it, I'd call it good, just not good as a nagura plate where you want the stone to shed nothing. It would seem that a pricey nagura with it is a bit redundant, because the stone is happy to give you what you'd get from tomo, and at least the slurry blackens wheras a very cheap stone would slurry too fast for any slurry to blacken.
His Koma may be harder than the base stone and yet the base stone can be just fine with a softer nagura. Koma vary in hardness and some can be very hard.