metal hardness is measured in rockwell hardness units , and the hardness of a blade ranges from 35 on the low end to 50 on the high end some blades may go higher. example is stens carbide tongsten blade has a hardness of 65. the higher the hardness value the longer the blade will retain a decent edge . you pay more for harder blades but you pay even more for the cheaper blades , by spending more time shapening and replacing them. i fond this in grounds maintenance magizine. so i whent to the barn to get some blades to see what thy are and couldnt find but one blade out of greatdane ,lawnboy, toro and snapper that has a two didgit number. the snapper has a square box next to the numbers 80k . is this rite do they make them that hard and why are the other blades not marked ?