A markedly heavy and hard bench stone. This is a jizuya for polishing the hard steel core of a knife, a razor or tool to a mirror finish representing the compacted grain of the metal. Sharp and shiny. Jizuya uchigumori is a type of stone that is homogeneous and void of alternate rogue minerals that can scratch the steel.
Jizuya is rarer than Hazuya but because of its structure and the fact that it is called upon to only polish the slender steel a large stone like this will last a lifetime. 
In theory finger flakes can be made for polishing small areas of slender edges. To harvest these the stone needs to be chipped. I would not recommend doing this.