For the "Flint" object in F&S we can use hundreds of types of rocks. Sandstone, granite, onyx, agate, jasper, dolomite, etc as well as many flints, cherts and quartzes. Carbide steel will also work. The key here is hardness. It must be harder than and sharp enough to scrape off tiny bits of the "Steel".

For the "Steel" object in F&S we can use several types of pyrites, titanium, steels from 1055-1095, A2, O1, O2, W1, W2 and L6. Improvised strikers such as shovels, chisels, putty knives, razor knives, Olfa blades, screwdrivers, files, fish hooks, rasps, rakes, hoes, saw blades (hand saw, bow saw, hacksaw, etc. Note: very few SawzAll blades work), axes, knives, golf clubs, etc. Of course some objects will work better than others, so experiment and see what works for you.