Bark

Tree bark was an organic material deriving from trees. It was an organic substance that coated the outermost layer of a tree.

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, 9 containers, ox spit, a biscuit, and a chicken poo. (TV: Demons of the Punjab)