When it comes to groundwork, there are several things you can use. A lot of people use a product called Celluclay. It's kind of an acrylic paste. There are several other similar products. As for myself, with my limited experience in dios, I actually use real dirt. I dry it up, grind it into fine dust, and sift it so that there are as few inconsistencies as possible. After that I mix with water and white glue and spread as groundwork.
As for other materials, virtually ANYTHING goes, as what you want is a lot of variations of colour and texture, just like in real life. It all depends on what you're building.