Try this code instead. I'm kinda sure stackpos does matter somewhat.
function onUse(cid, item, fromPosition, itemEx, toPosition)
rockPos = {x = XXX, y = YYY, z = ZZZZ, stackpos=1}
rockThing = getThingFromPos(rockPos)
if rockThing.itemid == AAAA then
doRemoveItem(rockThing.uid)...