Naniwa Chosera 800 Grit Japanese Whetstone Sharpening Stone, NO stand/base, 20cmx7cm surface, 2cm thick
This semi-coarse stone is perfect for grinding a new edge on your knife, and for working out nicks or cracks or other imperfections in old used/abused/damaged knives. Use this grade only when you regular maintenance and finishing isn't doing the trick anymore. It's relatively coarse and so will cut (remove metal) more aggressively and quickly than the higher grits often used in beginner stones/kits.
Naniwa Professional stones are widely considered the finest stones money can buy. They are the next generation of the superb Chosera stones, promising even faster cutting. They are artifical stones made with the magnesia bonding method. In terms of hardness they are slightly harder than the Super Stones, and because of the different bonding methods these "feel" harder and cut more quickly. They are very durable and long-lasting, but this extra hardness means that there is also more of a potential for cracking or chipping than with the Super Stones so be sure to air dry them and store in a cool, dry place and do NOT expose them to extremes of heat or cold (NO freezing!) or quick changes in temperature or humidity.
Do NOT soak them for a long time before use (5 minutes is plenty) and do NOT leave them soaking in water while you use other stones... in use, just splash more water on the stone as needed instead of prolonged and repeated soakings, otherwise hairline cracks can appear over time.