Green stain fungus on wood is caused by the blue-green cup fungus Chlorociboria aeruginascens and its close relative, Chlorociboria aeruginosa. It is a very common fungus, although it is more common to see the green stained wood than to actually see the fruiting bodies. They are members of the Ascomycota, in the family Helotiaceae of the order Helotiales. The green color is caused by the production of the pigment xylindein. Woodworkers have prized Chlorociboria-stained wood for centuries.