The mine it comes from is the Hakka mine and the particular layer is the Tomae layer, a more uniform and less stratified layer than the suita layer. This stone was sourced by our friend Shosui Takeda.
It absorbs water somewhat slowly, although adding more water from time to time is necessary. It is a muddy stone, unlike many other polishing stones, so it is particularly suited to single bevel knives like yanagis or usubas that require a refined edge and don't usually have a perfectly uniform front bevel. It is also well suited to double bevel knives like this Moritaka that has been ground to a zero grind - with no separate bevel but just the two sides of the knife meeting with just a slightly convex grind - not quite a full traditional hamaguri grind.
With continued sharpening, a continually refined mud is being built. I look at the edge as the process continues and a natural stone finish gradually replaces the initial synthetic stone finish, providing a contrast between the softer cladding steel (jigane) and the harder core steel or hagane, with the softer cladding producing a softer sand blasted hazy finish and the harder steel showing a type of mirror finish much like an antique mirror with a black haze. This is referred to as a kurobikari finish.
This Hakka stone is soft,and produces mud/slurry fast. I’d rate it 2.5. 5 being the hardest. It’s not suitable for razors but I bought it for sharpening my knives. It’s forgiving to use than harder stones (Suita,Asagi,Ozuku). Beginners shall find it easy to use.
It cuts regular German faster than Japanese steel like Kikuichi damascus. It gives a haze finish. So I’d estimate the grit around 10,000 which is fine and almost ideal for knives. Finer grit gives a sharper edge but difficult to maintain. Few hard hits against the chopping board can dull the edge easily.
Thanks to Mark for patiently answering my questions and giving a nice video review on a Sunday.
I gave it 5 stars - judging from size,performance relative to synthetic stones,and price when I bought it.
Really hard to come by stone now a days. Very muddy and soft but leaves a great finish. Does dish on the fast side but other than that a real pleasure to use.