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.
Yes you will need a special kind of rock. The spark is a tiny bit of burning iron struck off the source of iron by a really hard rock. So if you are only using rocks then you will need 1 iron rich rock and one harder rock. Iron Pyrite (fools gold) and quartz would be one combination.
Common practice is use steel as the iron source (high carbon knife or a dedicated steel striker) instead of a rock.
The tinder is very important. An iron spark is very short lived and not very hot (800 F) compared to what you get using a fire steel or ferrocerium rod (2000+ F). There is almost nothing in the wild that will take that cool spark and catch fire. Nor will cotton balls and lint work even with the addition of denatured alcohol to the material. The standard practice is to catch the spark in char cloth nested inside the tinder bundle. Fold the bundle over the smoldering cloth and blow until it bursts into flame.