The effort you take to sharpen a knife depends on the hardness of the blade that is used in the knife. If the hardness is more then it takes more effort to sharpen the cutting edges and the knife would retain the sharpness for longer time. If the hardness of the blade is less then it is easy to sharpen the knife and it will lose the sharp edge sooner. A Rockwell scale is used to measure the hardness of the metal that is used in the blade of the knife.