Description: Thallus crustose, episubstratic, grey, yellowish grey or greenish grey, rimose, granulose or verrucose, generally poorly delimited, without a distinct prothallus, up to 1.3 mm thick, with a chalky white, I+ blue medulla. Apothecia round to weakly ellipsoid, 0.3-1.2 mm wide, black but white- to grey-pruinose, sessile and constricted at base, with a flat to slightly convex disc and a thin, initially prominent, finally often poorly evident margin. Proper exciple black, extending below the hymenium; epithecium yellowish to brown-red, with numerous granules (polarized light!) which often penetrate deeply into the hymenium; hymenium colourless or partly pigmented, 70-100 μm high, I+ blue then reddish, except the upper part which is persistently I+ blue; paraphysoids branched and anastomosing, 1.5-2 μm thick,the apical cells hardly swollen; subhymenium pale brown, 20-30 μm high; hypothecium brown-black. Asci 8-spored, cylindrical-clavate, thick-walled, fissitunicate, with a thin K/I+ pale blue tholus and a small ocular chamber with an inconspicuous amyloid ring. Ascospores 3(-5)-septate, more or less constricted at the central septum, hyaline, oblong-ovoid, straight, 18-23(-25) x (5-)5.5-7 μm, with a thick gelatinous perispore. Pycnidia black, more or less immersed. Conidia straight, 5-8 x c. 1 μm. Photobiont trentepohlioid. Spot tests: thallus K- or K+ orange, C+ red, KC-, P-. Chemistry: erythrin, plus traces of gyrophoric and/or lecanoric acids.
Note: on siliceous rocks under overhangs and in other sheltered places; widespread in the Northern Hemisphere, with a few records from the Eastern Alps (Austria); to be looked for in Italy.