Spalting

Spalting is coloration and figure caused by fungi beginning to decompose a log, most visibly the dark, wandering zone lines that mark the boundary between competing fungal colonies -- a defensive chemical barrier the fungi throw up against each other, essentially fossilized in the wood. Left unchecked, the same fungal activity eventually turns the wood soft and structurally worthless; the trick to usable spalted lumber is catching it at the cosmetic stage and drying the wood before decay progresses further.

Spalted maple and birch, both pale species that show the dark zone lines dramatically, are the classic spalted woods sought out for turning and small decorative work -- the wood needs stabilizing resin in badly punky areas to be workable at all.