Hi - I'm just getting the hang of using chats and forums, and I'm sure I saw something about being able to slump glass over rocks (makes for interesting shapes/details) but can't find any info on it since (even try the trusty google isn't helping). Would anyone please be able to point me in the right direction? Thank you!!
An alternative would be to push rocks into some clay, and then pour pottery plaster/silica flour mixed 50/50 into the clay. Use the resulting plaster cast rather than rocks. Rather than clay, you could also use a silicone molding material for a more permanent mold for your plaster cast.
BE has excellent educational videos that talk about casting which might help.