Thanks Kevin for the compliment
MR ran an article a while ago called (something like...) "Rocks aren't hard", that gave some general information about what kinds of rocks are found in North America.
From a modelling point of view, sedimentary are easiest to do with the "slash carving", and igneous/metamorphic are (usually) better represented with some sort of casting. If you've got any more info give us some links... If you are up for doing an article/how-to, I say go for it! The Academy/Gems could always use info like this...
The compound I used is a "lightweight" filler, not a joint compound
per se. It comes in a surprisingly light container already premixed - you know you have the right one when it is the easiest to lift. It is almost like Cool Whip dessert topping, and dries in a foamy kind of texture.
Andrew