I have had great success sparking ferrocerium rods with almost anything sharp like a knife or rough like a file. I have heard that even a sharp piece of glass works well although I have not tried it. With the natural flint, quartz-type rocks that are plentiful where I live I can get a spark with hard stainless steel but it is a very tiny spark.... nothing compared to a ferrocerium rod. I don't know about other steel with natural flint (all my knives are stainless and I never carry a dedicated striker) but I hear that CS works well.