DC521 is a bench stone of a less common category, since it offers two different kinds of stones. The yellow part is a fast and aggressive diamond stone, while the dark is a slow, very fine ceramic stone. Both surfaces are precision flat, which is a prerequisite for perfect sharpening results. The diamond stone works fast and should be used if the edge is very thick. The ceramic whetstone works much more slowly and gives a smoother, razor-sharp edge when necessary.