reinforced plaster or "ludo" works well for many things.
5 parts plaster of paris (dry) 3 parts vermiculite and 1 part sand (all by volume) mixed with enuf water: pour into a mold made of clay or an oiled pan, remove when stiffened work before firming.
alternitavely: portland cement and fine sawdust plus some chopped nylon fiber makes an excellent and quite durable fake stone.
also: straight ceramic, in any one of many bodies
or: "air-hardening" clay with additions of powdered oxides or vermiculite works pretty well, but has brittle problems.
if you are looking for a rough look, unleavened dough shaped, baked and shellaced then painted can do some neat things.
check your local yellow pages under stone, many types of paving stone come in thin sections that look just like, well, stone
"rodding" can help with floors and ceilings, it isn't to difficult and can replicate most architecture styles.(email me for this one i can already here everyone falling asleep) as well as aquiring pallettes and making modular forms fron their cheap, rough wood.
zip me an email if you wish, i can maybe go more in depth...
