Bark

Martha Jones once saw faces in tree bark, water, on ground and on leaves whilst visiting a planet with the Tenth Doctor which had once been home to an advanced humanoid species. (PROSE: The Planet That Wept)

The Thirteenth Doctor requested tree bark as a material to make a "demon repellent", along with oil, saucepans, nine containers, ox spit, a biscuit, and a chicken poo. (TV: Demons of the Punjab)