Pyramid

Cut the faces of the pyramid out of foam-core board. Glue together with hot glue. Spread spackle paste over pyramid with putty knife, trowel smooth. When dry, etch in lines for stone blocks using straight edge and a pointed tool. Paint in a sandstone shade. Use a dark brown wash to shadow recesses. Done.

what scale....... cuz that might help,
small scale.....then clay would be ur least time consuming but if its BIG then cardboard, foam board, wood with clay on top would be best
-DJ

Easiest would be to make a cardbaord model and than cover it in clay and finish it of that way. Instead of cardboard you could also use foamboards or similar.

Bassically the same method as I use for making houses for Diorama.

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Here is how I make a nice stoneface wall, might come in handy for your project:

1.) Put a fairly even layer of clay onto your surface.
2.) Mark out the stones and remove clay that goes between the stones, can be fairly uneven.
3.) Take a piece of aluminum foil and crumble it into a ball(not too smooth), make a few.
4.) Roll the ball/s randomly over the stones and the edges, this should give a radnom pitted look.
5.) Leave to dry, and than paint and dry-brush the surface to respresent stone.

Being fairly random and too accurate here is the key to a good natural looking finish.

HTH.
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I built one from clay. it turns out pretty good, and when it's hardened you can airbrush it or whatever you want.