Burl walnut (American English) or Burr walnut (British English) is wood harvested from the base of walnut trees, where a large ball often occurs. The burl itself is actually an abnormal growth or deformity on the tree, often caused by some sort of stress such as a virus or bug infestation. When cut, the burl has a distinct appearance that can be finished to create an attractive piece of furniture or other structure. Walnut burl wood exhibits a rich dark color with deep, complex grain patterns, and is often used in crafting high-end furniture, veneers, and decorative items.