A burl (also called a bur or burr) is a tree growth in which the wood grain has developed in a distorted manner.  Burls are typically found in the form of a round outgrowth or protrusion from a tree trunk or branch.  The basis of burl development remains the topic of much debate.  However, most authorities agree that a burl usually develops when a tree experiences an environmental or induced stress (e.g., insect, mould or viral infestation, physical trauma, an excessively moist environment.)  Many burls grow beneath the ground and are attached to or are part of the root system, but they may also appear as single outgrowths or groups of bulbous protrusions on the trunk or branches of a tree.  They are often considered a malignancy on a tree and may not be discovered until the tree dies or falls.

Burls yield a very irregular and highly figured wood that is desired for its beauty by wood turners, carvers, sculptors and furniture/cabinet makers.   Almost all burl wood is covered by bark which adds to its striking appearance.  Its rarity and highly figured appearance also adds to its expense.  Some burl woods have dramatic, colorful, and highly figured unique patterns from the wood grain growing randomly.  These spectacular patterns enhance the beauty of wood sculptures, turnings, furniture, and other creative productions.  Some types of burl wood can be very hard to carve and turn based on the number of inclusions and abnormal gain patterning which may add to its fragility.

Burls may be very small or enormous in size and weight.  Some species of trees develop burls more readily (Giant Coastal Redwoods, Big Leaf Maple, several species of Oak) whereas other species rarely develop burls (South American Rosewood, Hickory, Walnut, Amboyna burl -- from the Asian Padauk tree). 

For additional information about burl origin, see:

http://www.aaes.auburn.edu/comm/pubs/bulletins/figureinwood

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