NAN-A-120
Naniwa Fine 220 Grit Japanese Flattening/Dressing Stone, Grooved, 30cm X 15cm X 2cm
Over time, any sharpening stone will start to wear and the surface will become hollowed out somewhat, and so it will need to be flattened (also sometimes referred to dressing). This stone is made for this purpose. You can wet the surface and rub the stone gently over the top, taking off the high edges and making the stone flat again. You can also spread a small amount of silicon carbide "sand" (see below, under "related products") on the sharpening stone to help the grinding and smoothing. This is a very large and heavy stone so it's useful for even pretty large stones, and includes rubber bumpers for the ends to help steady it while in use.
For casual home sharpening, you might also consider the much smaller (and so more affordable, and easier to handle and store) smaller version of this same stone.
Warranty: 60-day warranty against manufacturing defects.