Green elfcups (Chlorociboria aeruginascens) on dead wood in Stadsskogen, Uppsala, Sweden. It is possible that it is another closely related species, Chlorociboria aeruginosa, but microscopic examination of the spores is required to tell them apart.
Green elfcups are tiny mushrooms that grow on moist, barkless and decaying hardwoods, especially on oak. The fungus stains the wood blue-green. The pigment responsible for the blue-green color is a quinone called xylindein. Green wood, infected by the green elfcup, is a common sight, but the fruiting bodies are less frequently encountered. Infected wood, known as “green oak”, was once used in the production of Tunbridge ware, a form of decoratively inlaid woodwork, where small pieces of wood of different colors were glued together to form a mosaic.