Description: Thallus crustose, episubstratic, chalky white to grey-white, 0.3-1.5 mm thick, continuous to faintly rimose, frequently delimited by a thin, black hypothalline line. Medulla white, mostly I+ blue. Apothecia lecideine, dull black, immersed to adnate, usually angular, 0.5-1.5(-2.5) mm across, with a flat to convex, frequently umbonate disc and a well-developed, undulate, raised proper margin. Proper exciple green or green-black and N+ purple in outer part, paler within; epithecium green to green-brown or black-brown, K-, N+ purple; hymenium colourless to pale pink, rarely pale green, 50-80 μm high; paraphyses coherent, simple or sparingly branched and anastomosing, 1.8-2.2 μm thick at mid-level, the apical cells capitate, to 4.5 μm wide; hypothecium colourless to pale ochraceous brown. Asci 8-spored, narrowly clavate, thick-walled, with a K/I+ pale blue tholus and a strongly amyloid, thin apical cushion, surrounded by a I+ blue outer layer, Lecidea-type. Ascospores 1-celled (but with a conspicuous plasma bridge and appearing 2-celled), hyaline, blunt ellipsoid, (8-)10-13(-15) x 5-7(-8) μm, not halonate. Pycnidia rare, black, immersed. Conidia bacilliform, 8-12 μm long. Photobiont chlorococcoid. Spot tests: thallus K-, C-, KC-, P-, UV-. Chemistry: thallus without lichen substances.
Note: a circumpolar, mainly arctic-alpine, variable species (Hertel 2006) found on calciferous siliceous rocks, especially schist, in cool and humid situations near or above treeline; most frequent in the Alps, where it reaches the nival belt, but also present along the Apennines.