An aluminum pie plate in our garage is our firesteel testing ground. We've experimented with many materials. The dryer and fluffier the better. Milkweed, cattails and birch bark work very well, but birch bark is best if you scrape the inside of the bark into a powder.

I've never had success with grass or leaves. They're not hot enough and simply go out too soon, not allowing the fire to eat enough to sustain itself.