Then it hit me. I am painting over real rocks with several layers of paint, so that it will look realistic. It's a process that takes a fair chuck of time, to make it look real when it was real to begin with.
We do the same with metal bits such as suits of armor or iron tubing. I will prime an armored knight white/black and slowly work my way up to the silver the model started out as.
I know, it wouldn't really work out if we just left it, as is, and it's a lot easier to age or modify the color if it's primed to begin with. It just struck me as funny that some days it seems like I take a lot of steps forward just to go backwards.