Chlorociboria aeruginascens Kanouse ex Ramamurthi
Korf & Batra 1958 
Blue stain fungus, green elfcup, green wood cup
Turquoise-colored mushrooms? How cool is that? I’ve been looking for these for several years, and although I have often found the mycelium on wood—a blue-green stain—I have seen the fruiting bodies only twice, on Mt. Watatic and Oak Hill, both in Massachusetts. Returning to one of the places where I had seen the mycelium, the wood had been removed, apparently by someone who recognizes its wood carving value. It is used in creating intarsia, an inlaying process using tiny, thin veneers similar to parquetry. So I am grateful to Amanda Felegie for sending in the photo she took at Ricketts Glen State Park in Pennsylvania.
Identification: This fungus appears on decayed hardwood, principally oak, spreading deep into the wood. It has small cup-shaped asocarps attached to the wood by a stemlike base called the stipe. The fruiting bodies rarely exceed ¾" (1.9 cm) in size. The fruiting bodies appear only on wood that is decayed enough to be free of bark. Michael Kuo’s site, MushroomExpert.com, points out that these could be either Chlorociboria aeruginascens or Chlorociboria aeruginosa. The only way to differentiate between them is by comparing the spore size, which are a microscopic 5-8 µm by 0.7-2.8 µm, smaller than those of C. aeruginosa.
Chlorociboria aeruginascens on Wikipedia
Chlorociboria aeruginascens on Michael Kuo's MushroomExpert.com
Chlorociboria aeruginascens on Tom Volk's Fungi site, at the Department of Biology at the University of Wisconsin
Chlorociboria aeruginascens description by Thomas H. Kent, last updated 25 May 2020.