First of all: nice work, looks very promising

since Arachne already covered the line-problem with the tiling, I thought I could contribute with another small problem I saw in the tiling..

edit:
What I tried to show is that it seems that you have a bright area next to a dark area at the same height while one should try to avoid such areas. Of course you shouldn't pixel all stones which are at the same height in the same tones, but if they differ too much the tiling becomes visible.
My edit is pretty bad, but still I think one can see what I mean

It's not extreme in this case, but I thought this might explain a little what to look after when avoiding visible tiling.