Just for fun, how would I go about lighting a fire with naturally shaped rocks by striking sparks? It might be useful if I were to camp in the far north, where there are no trees. What should I use for tinder? Do I have to find a certain kind of rock?
Flint is your best bet and it sparks much better with steel than with a rock like iron pyrite. The better rocks you find, the less the tinder matters, but you will need something like a cotton ball or similar fine material that is very dry in most cases when you don't have actual steel and pure flint. Any old tinder will do - fungus, grass, wood splinters or mashed wood / sawdust.
Chalcedony (a chert form of quartz) and other flints are also very useful since you can make edged tools made of stone with enough skill and the proper raw materials. Jaspillite (or taconite as we call it in Minnesota) also will spark and has a high iron content.