Naniwa Chosera Series 400 Grit
The Chosera 400 whetstone uses magnesia as binder for the abrasive, in this case white aluminium oxide.
The Chosera Series stones are do not need to be soak with water. This makes Chosera stones rather easy to sharpen with. Because of their hardness you will not cut into the stone easily and sharpening with these stones will take less time. Their hardness make them a good choice for knives made from hard steel.
Splash with water before and during sharpening, do not soak or store in water.