A burl results from a tree undergoing some form of stress. It may be caused by an injury, virus or fungus. Most burls grow beneath the ground, attached to the roots as a type of malignancy that is generally not discovered until the tree dies or falls over. Such burls sometimes appear as groups of bulbous protrusions connected by a system of rope-like roots. Almost all burl wood is covered by bark, even if it is underground. Insect infestation and certain types of mold infestation are the most common causes of this condition.
Burls yield a very peculiar and highly figured wood, prized for its beauty by many; its rarity also adds to its expense. It is sought after by furniture makers, artists, and wood sculptors. There are a number of well-known types of burls (each from a particular species); these are highly valued
and sliced into veneers for furniture, inlay in doors, picture frames, household objects, automobile interior paneling and trim, and woodturning. The famous birdseye maple of the sugar maple (Acer saccharum) superficially resembles the wood of a burl but is something else entirely. Burl wood is very hard to work with hand tools or on a lathe because its grain is twisted and interlocked, causing it to chip and shatter unpredictably. This "wild grain" makes burl wood extremely dense and resistant to splitting, which made it valued for bowls, mallets, mauls and "beetles" or "beadles" for hammering chisels and driving wooden pegs.