Foam - solid core shaped verses hollow core framed would be a start.
Foam laminated or as a block and differences between types of it than surface preparation to hold paint or glues used to laminate it. Could even laminate together a bunch of cardboard than coat it with material from muslin to dutchman to fiberglass. Lots of coatings can be done to a solid ball. Depending upon the size, a pebble can be the inner cutting from a few sizes of hole saw on lumber. Fill the hole, use a knife and sander to make the pebble round or have shape etc. A band saw on lumber otherwise can make pebbles when used in combination with a belt sander. Than of course there is always bark chips when painted and actual rocks as other solid materials.
Hollow core such as a base plywood/lumber frame and shaped with chicken wire and other things. Than it’s covered by paper mache, dutchman, burlap or fiberglass. Advantages/disadvantages of each, various materials used etc. What type of frame to use and how much support for the wire is needed. What type of wire to be using in the first place?
Advantages/disadvantages of both methods and individual techniques in addition to ones others hopefully will recommend. What might work for some sizes of stone won’t for others so size/scale might be a determining factor. Can’t exactually stand on a paper mache stone unless it gets internal bracing. Yet once it has it, such a rock should support a person better than one made from expanded poly-styrene. Debatable about extruded poly on the other hand especially if it has plywood countersunk into it to distribute the surface pressure on it. What’s the budget, time and skill level, much less how long of a run/tour will it have to last? Is the rock moved between scenes or during the show if at all? Is it thrown, kicked, skipped across a pond...? Lots of little details.
Most of this stuff is in books, hit the library. Hope the general outline/jot list helps. If you don’t get the help on the forum you should get, perhaps post your rough draft to see if you can get more ideas on it and comments.