[3]: 222 Arthonia species are typically crustose lichens, forming thin, often inconspicuous patches that can be either flush with the surface (immersed) or sitting on top (superficial).

In some cases, however, a less well-known type of green alga is involved, and a few species are only weakly lichenised or even live partly by breaking down dead organic matter (saprophytic) or parasitising other lichens (lichenicolous).

The main spore-producing region (hymenium) typically reacts to the same chemical tests as the thallus hyphae, and these reactions help distinguish Arthonia from other lichens.

Thread-like fungal elements (paraphysoids) weave through a gel-like matrix, becoming more intricately branched and pigmented near their tips, where they often form small, dark "caps".

They are often semi-fissitunicate (splitting in a controlled way when releasing spores) and possess a large, domed apex with a specialised "ocular chamber" that may show subtle colour changes with chemical tests.