What about concrete lithops?

Many years ago I saw some pictures of some garden seating shaped like various lithops made from concrete. They were coloured naturalistically and looked great. Unfortunately I didn't get around to enquiring about cost quickly enough and never saw them again. My question is - how hard do you think it would be to make some? I'm sure one could carve polystyrene lithops big enough to sit on, but do you think one could then generate moulds from them to use for concrete?