Making A Cake Look Like Stone
Decorating By ReneeCake Updated 9 Mar 2011 , 1:02pm by PistachioCranberry
There are these chocolate candies you can get online called "river rocks" and they taste delish! 
Not exactly of your choice but you would like this cake.
http://www.mydish.co.uk/recipe/9515/malteser-layer-cake
http://www.google.de/images?hl=de&rlz=1G1GGLQ_DEDE318&q=fossil+cake&um=1&ie=UTF-8&source=og&sa=N&tab=wi
have you googled that yet?
http://cakecentral.com/gallery/1369529
there is one right here
That is really cool! You can do it in buttercream and ganache and either airbrush the different colors or tint the buttercream different shades. After you put it on the cake, you can use a brush or palette knife to get a good texture. HTH
Could you mix up different stone colors and and layer them in you bag so when piped it has that marble looking color?
Or, make rocks with royal icing and after the dry paint them rock colors?
Or, candy melts?
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