Spalting is a coloration in wood caused by fungi. Usually found in dead trees (but occasionally in living ones as well) spalting falls into three main types; the type of most interest to wood workers is called zone line spalting, characterized largely by winding thin streaks of dark red, brown and (uaully) jet black, and resulting from colonies of different fungus types interacting with one another or delineating themselves.
Structurally spalted wood is rarely used; it's use is strictly decorative. The fungus that causes spalting also causes wood to rot, and finding wood with notable spalting but little rot is rare indeed.