Jim,
Greg has it right.
Structolite is a darker plaster intended for use as a base coat when plastering walls. It is slow setting - overnight for most applications such as dipping paper towels for basic scenery. It dries to a rough finish that is perfect for rocks.
Good for just about all phases of scenery building, except for casting rocks in latex molds. But, it is great for blending rock castings or chunks of styrofoam into a hillside.
It's available at Home Depot in 50 lb bags for about $15. It has a long shelf life if kept in tight containers away from humidity.
Jim
