Hoary-looking blackish patches on hard stone/masonry mark Grimmia laevigata out as ‘something different’. Although it quite often grows with the likes of Grimmia pulvinata and G. trichophylla, it lacks capsules and doesn’t much resemble either of them.
With a hand-lens, you’ll be able to see that the leaves look dark green to blackish and opaque (because they are bistratose) and the lower nerve is not at all obvious. While the hair-point can be quite variable in length, it is very white-looking, rigid and spinosely toothed.
Microscopically, the leaves have an unusual shape (for a Grimmia), with a wide, sheathing base and a wide, concave appearance. Leaf sections (see below) are really distinctive and should easily confirm this species.Read the Field Guide account