A crustose species with a chalky white to grey thallus that is matt and may be cracked and areolate in the centre. The dark prothallus may be poorly developed. The innate apothecia are often arranged in distinctive concentric rings. The black disc is not pruinose, but may be surrounded by a slightly paler or white-pruinose margin.
Medulla: K+ yellow
In Scotland, Rhizocarpon petraeum is widespread on hard, siliceous rocks close to mortar, slightly basic rocks and walls, and coastal rocks. BLS distribution map
scottishlichens.co.uk 