Description: Thallus crustose, episubstratic, verrucose-areolate, yellowish grey to bluish-grey, sometimes delimited by a dark prothallus. Areoles 0.6-1 mm wide, usually rounded, convex, developing on a dark hypothallus; medulla white, filled with crystals, opaque, I-. Apothecia lecanorine-aspicilioid, 0.3-0.6 mm across, immersed, usually 1 per areole, with a black, epruinose, flat disc, and a very thin, persistent, often split thalline margin. Excipulum poorly developed, to 25 µm wide; epithecium olive green to olive-brown, N+ green, K+ brown; hymenium colourless, 120-150(-185) µm high, I+ blue or sometimes partly turning yellow-green; paraphyses in lower part 1.7-2.5 µm thick, in upper part slightly branched and anastomosing, submoniliform, to 3.5 µm wide at tips; hypothecium colourless, I+ blue. Asci (6-)8-spored, cylindrical-clavate, the thin outer coat K/I+ blue, the wall and apical dome K/I-. Ascospores 1-celled, hyaline, broadly ellipsoid to subglobose, 20-26 x 12-14(-18) µm. Pycnidia partially immersed, with a black, punctiform ostiole. Conidia filiform, 6-8 x c. 1 µm. Photobiont chlorococcoid. Spot tests: cortex K-, C-, KC-, P-; medulla K+ yellow turning red (crystals), but only after pretreatment with HCl. Chemistry: aspicilin, medulla with norstictic acid in low concentrations.
Note: on siliceous rocks wetted by rain in the mountains. The epithet gibbosa was frequently used by European authors for C. caesiocinerea, so that the presence of this northern species in the Alps is dubious.