Buellia saxorum

Buellia saxorum is a species of saxicolous (rock-dwelling) crustose lichen in the family Caliciaceae. It forms a moderately thick, white to pale yellow-grey crust with a network of fine cracks and a distinctive black border where it meets other lichen thalli, creating a mosaic pattern. The lichen's reproductive structures appear as small dark discs up to 1 mm in diameter on the surface of the thallus. First described by the Italian botanist Abramo Bartolommeo Massalongo in 1852, B. saxorum can be identified by its chemical spot test reactions due to the presence of atranorin and gyrophoric acid. It is found in several European countries including Britain, France, Italy, and Spain....read more on Wikipedia.