The
thallus of
Astrothelium rubrocrystallinum is , smooth, somewhat shiny, and continuous, covering areas up to 5 cm in diameter and about 0.2 mm thick. It is olive-brown in colour, surrounded by a black line approximately 0.2 mm wide, and does not induce
gall formation on the host bark. The ascomata are roughly spherical, measuring 0.30–0.45 mm in diameter, mostly found in groups or lines of 2–5 in poorly distinguished . These are mostly distinctly raised above the thallus and are of thallus colour. Inside, there is a dense layer of red crystals about 0.2 mm wide around the ascoma wall, visible as dark through the
hyaline cortex around the
ostiole. The wall is black all around, up to 50
μm thick. Ostioles are apical, not fused, flat, black, and surrounded by an ochraceous ring of about 0.2 mm, which sometimes has a grey margin. The is inspersed with hyaline oil globules.
Asci contain eight
ascospores each. Ascospores are hyaline, contains three
septa, and have a shape. They measure 22–27 by 7–9 μm and have rounded ends and diamond-shaped . They are not surrounded by a gelatinous layer. were not observed to occur in this species.
Astrothelium rubrocrystallinum belongs to the
Astrothelium annulare group and is characterised by the copious red crystals in a thick layer around the ascomata. It is most closely related to
Astrothelium buckii, but differs in having shorter with typical dimensions of 37–47 by 14–16 μm. ==Chemistry==