When it gets warm, go to your local creek and look for some chert or chalcedony (other forms of sedimentary silica like flint). I can find it almost anywhere in the Midwest. It is a common surface rock in most of the US. I'm not sure about where you live, but it is worth a look.

http://images.google.com/imgres?imgurl=h...%3Doff%26sa%3DG

Both chert and chalcedony are hard enough to liberate a spark from a piece of iron. Remember, it isn't the rock that sparks, it is the tiny chunk of iron scraped off the iron striker by friction that burns. The rock just has to be hard enough to scrape an iron spec free from the iron striker. Flint is hard enough, but there are other rocks that will do the job too, albeit with varying degrees of sufficiency.

BTW, flint is a type of chert.