Rhizocarpon badioatrum spreads as a crust up to about 10 cm across. Its surface is broken into tile-like plates (), each no more than 2 mm wide. These plates are , medium- to dark-brown, often tinged pinkish or red-brown, and sit within a well-developed jet-black border of fungal tissue, the . The areoles are thick and mostly flat but can have scalloped edges; where they meet one another the thallus appears as a mosaic. Internally, the white fungal layer (the
medulla) shows no iodine
staining reaction (I–), and a drop of
potassium hydroxide solution (the
K test) may leave it unchanged or briefly yellow. Scattered between the areoles are the fruit-bodies (
apothecia), black up to 1 mm in diameter that remain essentially flat. A narrow, dark wall called the surrounds each disc; in section the inner part looks red-brown and flashes purple-red with the K test. The very top layer of the disc (the ) is likewise red-brown and gives the same K-induced purple tint, sometimes releasing tiny red that dissolve in the reagent. Beneath this lies a clear spore-producing zone (the
hymenium, about 110–120
μm tall) and a medium-brown foundation layer (the ), which shows no colour change with potassium hydroxide. Each club-shaped
ascus produces eight
ascospores; the mature spores measure 27–38 μm by 13–19 μm, are divided by a single
cross-wall, and darken through blue-green to deep brown as they age.
Para-phenylenediamine (the Pd test) is usually negative, though an orange flash or traces of
stictic acid are occasionally detected. ==Distribution==