Unknown Mdeium-High Grit Japanese Natural Sharpening Stone 4lb. 13.4oz. (2191.2g),
It looks more like perfect shaped stone.
I do not know which mine came from, but it might be medium grit stone around 1000 grit.
It raises great amount of dark gray color slurry quickly.
It looks not too soft and not too hard ideal hardness stone to me.
(Scale; 1 to 10, 1-3 is very soft muddy rough grit whetstone 3-6 is soft medium grit whetstone, 6-8 is hard medium grit whetstone; 8-10 is fine grit whetstone)
It makes my blue II steel kasumi blade knife Jigane became frosted silver color, and Hagane also became silver frosted color.
It is hard to tell, but it might be entry class fine grit stone.
(Scale; 1 to 10, 1-3 give deep scratch marks to the steel, 3-5 is delete the 1-3 scratches, 5-8 makes 3the steel cloudy, 8-10 makes the steel semi-mirror finish or mirror finish)
Natural whetstone has wider capacity than artificial whetstone, and actually, we do not know until sharpen blade on the hone for a while. Because artificial whetstone molecules do not break down to smaller sizes, but Natural whetstone molecules are possible to break down to smaller sizes, and those molecules are mixed with sharpened steel molecules, and make Jigane as suitable Jigane color, and Hagane as suitable Hagane color. It is natural mystics.