Since you mentioned wanting it to be lighter than rock, your best option would be to experiment with foam, or Vermiculite and Perlite. YOu can replace the aggregates in cementatious mixes with the Perlite or Vermiculite and have the strength of cement without the weight of sand/gravel.
Another option would be to simple put multiple coats of a mortor mix over a foam body. It takes practice to get the rock not to slough off, and it can be very tedious and time consuming. The beauty of it though is you can built it any size you want...make it solid...make it hollow. Hell, you can even give it a ramp up the back if you have a handicapped chuck named Hooti. The only limit is you imagination.
Texture techniques are also unlimted..you can texture anything...just make sure the rock is solid enough before doing it.
I have learned an immense of information for JPJ technologies (look them up), and also ezyrock.com...both offer different methods of faux rock construction.
Best of luck,
John E.