A wood burl happens when a tree has a growth or deformity due to infestation or illness (some kind of stress). It's found in oak, maple, walnut, cherry, ash, redwood, and other tree species. It's usually found in the roots, curled branches, or large rounded “warts” on the tree.
 
Amboyna Burl, Pterocarpus indicus, is the World's rarest and most expensive burl, possessing incredible colors and patterns. It was used for the first Rolls Royce's dashboards.