Well no-one got it fully right, though TW's 'close-up of a rock' was near, but I'm giving the winner's award to Jordandog for 'mosaic in nature'
They're actually lichens on a large rock on the west coast of Ireland. I've no idea what the dark lines are; maybe some sort of boundary produced by individual lichens to prevent others from overgrowing them - just a guess. Lichens have a symbionic relationship with a fungus, so the dark lines may even be the fungus; anyway definitely not 'inked in' I assure you!
Here's a link to the full size image, and as you can see I placed a bunch of keys on the rock to give scale. You can make the image larger still by clicking 'Full size' just above it.http://s43.photobucket.com/albums/e357/p...nt=Lichens2.jpg