The thallus of Arthothelium spectabile is mostly immersed in the surface of its substrate and appears as creamy-white to grey-white patches. It is often edged by a thin brown boundary line (a prothallus) that helps define the extent of the thallus. The photosynthetic partner is a trentepohlioid alga.
Its fruiting bodies (apothecia) are black and lack a powdery coating (they are not pruinose). They are typically rounded to irregular in outline, sometimes becoming angular (polygonal), and may be either circular (up to about 1.2 mm across) or somewhat elongated (to about 2 mm long). In section the apothecia are about 120–150 micrometres (μm) tall. The upper surface layer (epithecium) is red-brown and turns a deeper reddish colour in potassium hydroxide solution (K+). The spore-bearing layer (hymenium) and the underlying tissue (hypothecium) are pale reddish-brown, showing at most a faint greenish reaction in K. Numerous fine paraphysoids (about 0.5–1 μm thick) run through the hymenium; their tips are usually unpigmented and do not form swollen caps. The ascospores measure 26–36 × 12–15 μm and are ellipsoid to somewhat cylindrical-ellipsoid. They are muriform, with 5–7 cross-walls, and most of the compartments are further divided by 1–3 longitudinal septa.[4]
