A burl of the North American hard maple. A burl is a tree growth in which the grain has grown in a deformed manner. It is commonly found in the form of a rounded outgrowth on a tree trunk or branch that is filled with small knots from dormant buds. A burl results from a tree undergoing some form of stress. Burls yield a very peculiar and highly figured wood, one prized for its beauty by many; its rarity also adds to its expense.