What type of stones are best for Istinjaa استنجاء?

What type of stones are best for Istinjaa استنجاء? Presumably they should be very very *very* slightly rough for traction: https://en.wikipedia.org/wiki/Asperity_(materials_science) ... but of course not too rough. Is there any Sunnah on the particular types, qualities, grades & cuts of the stones? Any records of a particular type and cut of stone being manufactured for istinjaa? Would stones from a riverbed suitable for skipping be ideal? (Germane: I studied metallurgical science in University, so the question is being asked from a Sunnah standpoint, an engineering standpoint, and an end-user standpoint.)