The quest is repeatable, but you can't be doing the Inspector's Gadget quest at the same time. It will get you 200 gil every time you do it after thie first.
I've been playiing this game for ever and gotten about 20-30 rocks and not ONE has been the right rock... this is really annoying... guess i'll keep on trying. :-P
I only got 2 smooth stones out of all of the crawlers I have killed ever...which would be quite a bit. Luckily, the 2nd stone worked for the In a Pickle quest.
I believe this is a pretty rare drop...although not as rare as the meterorite.
i windurst north from the lv. gate(east) is a small pond... arround there u may found lots of crawlers.got two drops in 5 kills. both stones too light :))