Neoplaca mirabilis forms a distinctive crust-like growth (
thallus) composed of scattered scale-like structures () that are typically whitish to greyish in colour, though occasionally showing a pale yellow tint. These squamules measure 1–4.5 mm across and 0.3–1 mm in thickness. As they develop, they begin as tiny about 0.1 mm wide, which then grow into small convex structures and finally mature into fully formed squamules. These mature squamules often display an intricate pattern of wide, winding folds on their surface that resembles brain tissue. The squamules can grow individually or overlap like roof tiles. In some cases, they merge to form larger, patterned patches up to 1 cm across. A distinctive feature of the species is its method of
vegetative reproduction: it produces small, round structures called along the edges of the squamules. These blastidia are
citrine to orange-yellow in colour, creating a marked contrast with the paler squamules. Mature squamules are often surrounded by numerous tiny new growths, which likely develop from these blastidia, suggesting the species reproduces effectively through these structures. No other reproductive structures (
apothecia or
pycnidia) have been observed. When viewed in cross-section,
N. mirabilis reveals several distinct layers. The outer protective layer () is 33–40
μm thick and consists of two distinct parts: a lower colourless layer and an upper layer containing cells with brown pigment. A white, powdery coating () covers the cortex. Below the cortex lies a layer of algal cells (), 5583 μm thick, while the inner layer (medulla) is white and loosely structured, ranging from 0.2–0.8 mm thick. Unlike most members of its family, which produce orange-red pigments called
anthraquinones,
N. mirabilis contains different chemical compounds. It produces
simonyellin as its major
metabolite, with trace amounts of
consimonyellin, as well as an unidentified brown pigment with some characteristics similar to
melanin. These pigments are found inside the cells rather than on their surface, which is unusual for lichens in this family. When tested with
potassium hydroxide solution (K), the lichen turns orange-brown, and with
para-phenylenediamine (P), it briefly turns yellow. ==Habitat and distribution==