I use a lot of different rock molds from various manufacturers, including Woodland Scenics, but I just came across a rock mold I made a while ago from a fine grained piece of rock  found in the garden.

Latex rock mold

Nice detail in the rock and think the cast from it is going to make a nice outcropping in the river valley. Just needs some coloring and weathering plus some small evidence of vegetation.

Hydrocal Plaster cast made from latex rock mold