Last week I had some fun lettering on a few rocks I had picked up in the yard. The few accounts I had read by people about this seemed to indicate that it was difficult. It's not. It's actually rather ... I was going to say "absorbing" but that would be silly. It's true that if I didn't mix the gouache thick enough, a line of dark appeared around each letter; this disappeared as the fluid dried. Rocks are actually quite lovely to write on. I tried sumi ink (Moon Palace), gold ink (Spectralite), gold watercolors (pans), white ink (Dr. Martin's), and gouache (Schmincke mixed with either extra gum arabic or glair; I didn't keep track of which container was which). They were all lovely to work with.