Artist's conk

Ganoderma applanatum (Artist's Bracket, Artist's Conch, or Flacher Lackporling; syn. ) is a bracket fungus with a cosmopolitan distribution.
The spore bodies are up to 12–16 in across, hard, woody-textured, and inedible; they are white at first but soon turn dark red-brown.
It is a wood-decay fungus, using primarily dead heartwood, but also as a pathogen on live sapwood, particularly on older trees. It is a common cause of decay and death of beech and poplar, and less often of several other tree species, including alder, apple, elm, horse-chestnut, maple, oak, walnut, and willow.
A peculiarity of this fungus lies in its ability to be as a drawing medium for artists. When the surface is rubbed or scratched with a sharp implement, it changes from light to dark brown, producing visible lines and shading.