In Nylander's brief description, he mentions that Pyxine petricola has a slightly blue-green thallus, spreading thinly and attaching closely to surfaces.

Its reproductive structures, apothecia, are black, flat, and edged, with dark, oblong, two-compartment spores.

The thallus may be smooth or slightly convex, with pseudocyphellae—specialised pores allowing gas exchange—visible as white lines along the margins and upper surface, sometimes forming a network (reticulate).

Unlike some lichens, Pyxine petricola does not develop isidia (outgrowths for reproduction and dispersal) or pustules, and may or may not be covered in a fine powder (pruina).

The outer layer (cortex) of the lichen fluoresces yellow under ultraviolet light but is K- and Pd-.