Well I got frustreate one day using a ferro rod to do a fire and did it with one of those small razor knifes and got as much if not more sparks then with a carbon blade. Ferro rods work by scrapping off the alloy of the ferro material not by scrapping off the steel of a high carbon steel blade when using flint. I know the teeth of a hacksaw blade works better then the back of a hacksaw blade, it is harder, for making lots of sparks. So it is most likely what ever is harder, rockwell number, probable will work the best.