Flint shard

It is not a fourth material of the wood: it is what was already on the slope and nobody was picking up. The bank between the lion and the she wolf is scree, and on scree you find shards. It serves arrows and nothing else, which ties it to the place you found it rather than to a smith’s bench.

Nine small piles of one to three shards along the slope, and then nothing until broken stone turns up again: ten down the Minotaur’s landslide in the chapter of the walls, six on the fifth bank of Malebolge. Before those were sown, from the second chapter on there was not a shard or a branch left on the ground, and the game’s two recipes could only be made in the first.