I take fondant and shape the "rocks" then let them dry.
use whatever color and give them a wash of color then paint in any cracks and markings to resemble a river rock. Splatter color on with a brush to get that speckled look. Ga picture or a real rock to go by and good luck!!!!! 
lilie
I add alittle black coloring to some fondant, or corn flower blue will work too. and i just mix until its a marble then just mold pieces into rocks. Just don't over think it. If you need some dirt to go with the rocks, then just crush some oreo cookies and sprinkle around the rocks, its very effective look.
There are also candy that they make that looks just like rocks. They have the shine and all like a river rock.
I used these on some cupcakes. They come in a variety of sizes and colors. I bought them at Sugarcraft. They taste like M&Ms and look a lot like river rocks.
I use rolled buttercream. Make each rock individually adding black or brown food coloring and mixed only until you get the marblized effect you want. This picture has large rocks on it; but I've made smaller ones too.
P.S. Wear gloves when rolling the color into the rolled buttercream. (You should anyway for safe handling; but you had better so you don't get black fingers!)
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