I know what you mean, I know what works, but am sadly lacking on a lot of different kinds.
If you think there may be some around, just take a piece of harden steel with you (hack saw blade or piece of old file will work), on "walk about", when you see something promising, just strike it and see if it sparks.
I tried a couple of pieces of pyrite, (fools gold), just striking them together they will spark as well.
This was tip from Ala (spelling?) of Clan of the Cave Bear, fame.
At a rendezvous being held on a old trading post site, it being my turn to dig the fire pit, I dug up a piece of flint.
Used it to start the camp fire for the weekend, still have it.
Whether it use left over from the old post, or just natural, don't know, but was kinda cool digging up a fire-starter.
