In the dark cracks there are little random pixels that have a very similar color, drop them. Everything seems very random and messy, like the random pixels everywhere in the piece. I also think this piece could benefit from some depth, which could be accomplished by adding darker pixels at the 'intersection' of the cracks between the rocks. There is also a problem with it tiling... you should try your best to make all the cobblestones the same size, so you avoid having ones that stand out. Right now you have two very large ones that stand out, and then a line of small ones as well.