If you want to make stone wall or pavement, probably the easiest way is carving the stones in insulation foam. You can make individual blocks also, but you will need thicker foam.

Loads of really good 'how to's' on youtube.
Ideally use high quality modelling foam like Balsa Foam or AK Interactive Carving Foam.
Cheers
Derek

Second that. And if you're Dutchy like me-that is, frugal-you don't have to buy a whole sheet of foam insulation. Look for a building site your area and see if they've got scraps left over. I'll ask the crew if I can take some, and they're always interested to hear what it's for.

Prost!
Brad

Gypsum, plaster, alabaster, clay, but without sand impurities, in scale more like stone than foam.

Those can also be applied over a foam core, too. I've used foam insulation carved to rough shape, to coat with thinned wood putty, for diorama pieces, like a stucco-coated wall, cast concrete pieces, etc.

Prost!
Brad

Another option is cork sheet of the right thickness. Cut in strips, chop strips in blocks. Especially suitable to imitate worn masonry.

Adrian

Yes! I've seen modelers make walls that way.

Prost!
Brad

Yes, plasterboard (Drywall) offcuts were the "go to" source for making blocks for a long time.

If you have a lot to do then there are options for buying moulds to use with dental plaster (or similar):
http://www.hirstarts.com/molds/molds.html

Jumping in rather late, please check out this YouTube video, and check this guy's other videos as well:


I had nice results with insulation foam I found locally and with normal scalpel and straightedge (I don't have a foam cutter).
Another material is foam PVC, which you can get from the trash bins of sign makers. Some in my area were just handing me their leftovers for free.

Benny