Natural polishing stone used for Omote-zuri before Tsushima.
Omote-zuri is a work to smooth the surface of the polishing stone.
Polishers always use natural stone for this work, because if any particle of artificial stone remains on the polishing stone, it will scratch the blade.
Binsui is coarser than Tsushima, so some polishers use it before Tsushima.
Dimensions : 7 x 5 x 6cm.
