Lecidea lithophila (Ach.) Ach.
Thallus crustose usually thin, rimose to cracked and areolate, greyish, grey-blue, bluish, very often with areas and even entirely colored with rusty or brown-orange on iron-rich rocks, rarely delimited by a thin blackish hypothallus. Apothecia immersed at first, then sessile, 0.5-1.5 mm diam., disc flat to slightly convex, more or less pruinose, brownish, brown-blackish when moist, blackish when dry, exciple distinct, thin to thick, persitent, concolourous though lower surface is brownish. Ascospores narrowly ellipsoid, simple, colourless, 9-15 x 4-7 µm. Photobiont: Trebouxia. Thallus: C-, K-, KC-, C-, medulla: I-. Common, on iron-rich humid siliceous rocks, very common on schist slates and in slate quarry refuses but the most well developed specimen thrive on iron-rich moist siliceous rocks in humid pathways. See Lecidea lithophila forma.
N.B. This species is subjected to various interpretations according to different authors. Considered here sensu lato. Extraordinarily variable, may include several taxa for which identification requires chemical spot tests. See also Porpidia cinereoatra and Porpidia crustulata that also are I-.
N.B. As a curiosity and to show its remarkable adaptation to iron in its substrate, see: Lecidea lithophila forme sur métal (Fer).