Diplotomma

Diplotomma is a genus of lichen-forming fungi in the family Caliciaceae. The genus has a widespread distribution and contains 14 species. These lichens form firmly attached, crust-like patches that range from pale to dark grey and can appear smooth or cracked into an irregular mosaic pattern on their substrate. They produce black, disc-shaped fruiting bodies that start buried in the crust and later emerge to sit roughly flush with the surface, often dusted with a greyish-white powder....read more on Wikipedia.