Many people seem to refer to any highly figured wood or interesting grain pattern as a burl (also referred to as a burr outside the US), but I'm told "true" burls are extremely rare. What is a burl, and what causes it?
A burl is an outgrowth on a tree:
source wikipedia
They are cause by infestations or injuries.
Inside the burl the grain is twisted and interlocked, this makes it interesting for carvings and for its strength as it's less likely to split. This grain pattern also makes it difficult to work with.
answered Mar 26 '15 at 10:08
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The tangled grain pattern can also be very attractive, making burls popular with woodturners and folks doing decorative work (though for many purposes it's easer and cheaper to use burled wood as veneer over something more affordable such as plywood).– keshlamMar 26 '15 at 13:59
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2It should be pointed out that burls are quite common on certain tree species, for example maple, cherry, oak, and walnut here in the US. However, finding a "true" burl on a fir, pine or other coniferous tree would be quite rare to my knowledge.– cathodeMar 27 '15 at 3:38