Had I known I was photographing something uncommonly encountered, I would have made more of an effort to get an exceptional photograph or two. Michael Kuo, the principal developer of the excellent MushroomExpert.com states: "I had wanted to find the fruiting bodies of Chlorociboria for many years...the tiny mushrooms are seldom seen...This year I finally found the little blue-green cups, after years of searching." My father spotted these growing on the rotting log of a fallen ash tree (I believe) while I was photographing a nearby Ramaria in October of 2008. I was immediately intrigued by the colour, as bluish-green is rare in nature (on land, anyway).
The common names of green stain fungus, green wood cup or green elf cup are applied to the two species of this genus found in North America (seventeen species of the genus worldwide, centre of diversity in New Zealand). Chlorociboria aeruginascens and Chlorociboria aeruginosa are not restricted to North America, though, as they are also found in other temperate regions of the world. It is extremely difficult to tell the two species apart by sight-identification (as noted by Michael Kuo: Chlorociboria). Identification with a high degree of certainty is easiest via a measurement of the diameter of the spores using a microscope.
Jessie Glaeser and Tom Volk provide an excellent account of Chlorociboria aeruginascens. Of particular interest is that the mycelium--the part of the fungus that grows in the substrate and forms the bulk of its mass--stains the substrate (wood) a green or bluish-green colour. This is done via the production and deposit of xylindein, a naphthoquinone. The green-stained wood was and is used by woodworking artists, occurring at least as early as 14th century Italy in intarsia panels (photographs on Tom Volk's site). Tunbridge Ware, popular in the 18th and 19th centuries, also made use of Chlorociboria-stained wood in the intricate designwork inlaid in these wooden objects.