I 've had some success with granite and quartz, in some cases attached to each other, I've also succeeded in getting a spark from cullet (old remelted glass) using the back of my knife in all cases, anything that will shave a thin sliver of metal from your knife will work.
What I have found is that very few factory manufactured knifes will create a spark, (we tested it in Sweden on a survival course) but quite a few member made blades will, my SC_Barker clone was great at creating sparks from the above, in actual fact with a piece of natural flint and some charcloth it made both the signal fire and the cooking fire on that course, (I was lucky enough to be chosen, the depth of knowledge and expertise of the people on that course still astounds me).
I have also created sparks from natural flint and iron pyrites, but havent tried it with charcloth as yet, it should work, the pyrites creates quite a few sparks, although they are no where near as hot (bright) as they would be from steel.
Bottom line dont be blinkered by "Oh you have to use flint" like the man said get out and try various stones, all I would suggest is to make sure the "steel" you use does create sparks, so test it first, then experiment.
regards
Stephen
