Burl is a beautiful kind of wood. Ever looked at a woodcarver's wares and seen some object, a bowl, tray, or tobacco pipe made of wood with a dense and twisting grain? That's burl.
Some causes are:
Any way it starts, it causes a massive haphazard growth of cells (much like cancer), that cause twists and knots in the wood. This is what gives the pattern burl's prized for!
- Burl wood is usually darker than the characteristic color of the tree
- Since the grain isn't coordinated, it cannot be relied on to be strong (as tobacco pipe mentions, burl is hard, but hard != strong)
- Burl is difficult to dry without cracking
- Burl comes in all shapes and sizes, from a ping pong ball size to 1m by 2m!
Burl is obtained though (gasp) wood dealers, and if you google around, there are many boutique websites specializing in burl.
Burl is harvested by professionals. Some info and pictures here (including how not to harvest burl): http://www.xmission.com/~burlturn/Whyitcost/whyitcost.htm One of the more interesting things on that website, is that people hunt burl, and cut it off of trees, leaving the tree intact and healthy!
Info from http://www.hobbithouseinc.com/personal/woodpics/_figure.htm#BURL (there are some good pictures there too! Please /msg me if you have additions/suggestions/comments.)