I saw a post on another forum about making caves with a kind of slate, i dont want to make caves but i thought i could mould my own rock shape with small part of this slate, i was wondering if anyone knew what kind of slate/rock can be used, it was kind of grey and when smashed it shattered into thin wide bits.

Any idea?
Or any other rock/slate i could use silcone to mould my own shape with?

Thanks
Scott