A dark greyish-black, flat crust on recently dead or felled Beech trees.  The stroma erupt through the bark and the peeled-back bark remains as fragments  around the stroma.  There are small conical pores or ostioles, through which the spores are eventually ejected. 

The Common Tarcust Diatrype stigma occurs mainly on oak.  There are several other species of tarcust.

Photograph in its habitat, and close up to show pores; note substrate and associated tree species.

Mainly on recently dead or felled Beech trees

All year round