Thallus crustose, leprose, diffuse, 100-500 µm thick, pale to mid blue-grey in herbarium, brighter and greener in the field. Granules fine, 60-120 µm diam., ecorticate, without projecting hyphae. Lower part of thallus of decolourized granules but true medulla absent. Photobiont a green alga, cells spherical, 8-15 µm diam., sometimes divided into autospores.
Chemistry: zeorin and usnic acid. Spot test reactions of acetone extract on filter paper: K-, C-, KC+ yellow, Pd-. The KC reaction can also be observed by swabbing the spot of K from the thallus onto tissue paper and adding C.
The bluish green leprose thallus lacking a true medulla and without projecting hyphae, growing on calcareous walls, help to identify candidates for L. calcicola. The KC+ yellow reaction (indicating usnic acid distinguishes L. calcicola from other leprose British species (except for Lepraria ecorticata which however has a distinct pale yellow tinge). The presence of distinct haustoria help separate this Leprocaulon from Lepraria species of similar appearance.
The fine, bluish granules and lack of medulla gives L. calcicola a resemblance to Lepraria incana or L. ecorticata (though the latter has a distinct pale yellow tinge in the field which is absent in L. calcicola).
Known from a small number of localities in S.E. England (especially East Anglia).
On vertical or near-vertical surfaces of old mortared walls, on shaded or sunny surfaces; often growing directly on mortar, but also overgrowing mosses (both healthy and decaying) and flints.