Something dry and seasoned. Ray says "in particular wood that is white", but I don't quite see why it would matter. I believe willow works well, and I've had success with pine (although they may have been treated).
[edit] Have managed a couple of times to get a feather stick lit with a firesteel, but it wasn't really worth the chunks of firesteel that were scraped away to get that final magic spark.
[edit] Have managed a couple of times to get a feather stick lit with a firesteel, but it wasn't really worth the chunks of firesteel that were scraped away to get that final magic spark.