A few years ago in MR magazine there was a guy who build really nice, loose stone (no mortar) foundations on some of his buildings. He made a mold that would cast plaster strips of the appropriate size, once they were set he just broke them off to the sizes he wanted and stacked them.
Might not meet your requirement of "not heavy" though, if weight is an issue I've seen people use foam for stone-work, they carve the mortar lines in it, then spray it with nail-polish remover, watch it for the texture they want, then spray water on it to stop the dissolving. Takes practice, but makes really believable stonework.