The outer tissue layer of the apothecium, known as the ectal excipulum, has a delicate tomentose surface composed of hair-like, straight or sometimes coiled, smooth hyphae.

The abundant paraphyses, which may be entwined, are 55–95 by 1.5–2 μm, filiform, and septate with an unswollen, unbent apex that often extends beyond the level of the asci tips.

[12] This species contains a quinone pigment called xylindein,[13][14] a dimeric naphthoquinone derivative, whose structure was determined by spectroscopic means in the 1960s[15] and later confirmed by X-ray crystallography.

[16] It is this compound that is responsible for the characteristic bluish-green stain of wood infected by this species, used today in decorative woodworking such as Tunbridge ware[17] and parquetry.

[18] The use of this wood, known as "green oak", goes back to 15th-century Italy, where it was used in intarsia panels made by Fra Giovanni da Veroni.