Description: Thallus crustose, endosubstratic or thinly episubstratic (to 0.3 mm thick), continuous to finely areolate, white to yellowish-ochre, sometimes (especially in high-altitude endolithic forms) with a pale blue tinge. Medulla white, I- (rarely I+ pale blue). Apothecia lecideine, adnate and constricted at base, black, epruinose and somehow shiny or finely bluish-pruinose, (0.2-)0.5-1.5(-2.5) mm across, with a flat to finally convex, sometimes indistinctly umbonate disc, and a thick, usually persistent, often wavy proper margin. Proper exciple c. 60 µm wide, black, the hyphae 5-9 µm thick, K-; epithecium blue-green or greenish brown (the two colours often present in the same apothecium), 12-25 µm high, K-, often with a layer of crystals; hymenium colourless or greenish in upper part, 65-130 µm high; paraphyses coherent, branched and anastomosing, 1.7-2.2 µm thick at mid-level, the apical cells to 3 µm wide; hypothecium green- to brown-black, the upper part more intensive green, K- or K+ reddish brown, c. as high as the hymenium. Asci 8-spored, clavate, with a I+ pale blue tholus and a central amyloid tube, approaching the Porpidia-type. Ascospores 1-celled, hyaline, ellipsoid, 11-28(-33) x 6-13(-18) µm, halonate, the perispore to 3 µm thick. Pycnidia black, immersed, globose. Conidia bacilliform, 3.5-9 µm long. Photobiont chlorococcoid. Spot tests: thallus K-, C-, KC-, P-, UV-. Chemistry: without lichen substances.
Note: a cool-temperate to arctic-alpine, circumpolar species found on limestone and dolomite, more rarely on other calciferous rocks (e.g. sandstone and schist) in upland areas; one of the most common calcicolous species above and near treeline, throughout the country.