Reshaping its cutting edge by grinding away tiny amounts of the blade. And while it's the best way to sharpen a knife, all that grinding can leave the knife's edge rough and uneven. Using a knife steel, also called a sharpening steel or honing steel, helps smooth out that roughness, leaving a nice, straight edge. The best steels are diamond-coated, which can be used for both metal and ceramic knives. The sharpening material always has to be harder than the material the blade is made of.