I have found that if you pick something up & then put it down, it will evaporate. It could last the whole game if left alone, but once it is in your inventory, even if it put back right where it was, some property has changed. Pick up a weed in a town, put it right back, & come back the next day & it will be gone. Big stones don't last any longer, even if some pedestrain hasn't made off with it. Try this with one of Theo's boulders.
so that's where te fifth stone is... er or where it's not or why it's gone or argh whatever... damn bug