Many years ago, I started casting rocks using Plaster of Paris. Lately, I have been casting rocks  with Lightweight Hydrocal to save some weight. (A couple dozen rocks can add a lot of weight to a portable layout.) Most recently, I have tried using Lightweight Spackle in regular rock molds. It came out surprisingly well.

I wetted the mold well with 'wet' water (a few drops of dish-soap or alcohol in the water) and pressed the Lightweight Spackle into the mold with a small spatula. After drying, I had some very light rocks, with fairly good detail. They are slightly flexible, and can easily be carved and trimmed to fit. Acrylic paints or washes don't soak in like Hydrocal, but a little blending comes out looking pretty good.

Detailed mold texture does not come out as rough as with Hydrocal but, as with any rocks, a light sprinkling with a fine ballast or finely-sifted sawdust before or after painting ends up with some dramatic results.