There’s the tooth of the sperm whale (used in scrimshaw), white tree bark (birchbark), and cave art on rock walls.
One of the finest natural art materials is called Artist’s Conk or Artist’s Fungus (Ganoderma applanatum). It grows on the side of rotting deciduous trunks throughout North America, and can reach sizes of up to 20 inches wide. It has a gray, furrowed crust on the top surface, resembling a horse’s hoof, and a clear, smooth, milk-white under surface. 
The white surface bruises under the touch of a sharp nail. Artists can etch extremely detailed drawings on the wide, arching shape. As the fungus dries, it becomes hard and lightweight, and the drawing naturally darkens and cures to a solid, permanent texture like wood.----
More about preparing and collecting.
Thomas Volk, Mushroom expert's site.
Information from the National Audubon Field Guide to Non-Flowering Plants.