An empty runestone is 3gp and can be used to place a rune (fundemantal or property) in it for later transfer to a weapon or armour. If you take a rune from a runestone, the process cracks the runestone making it useless. So it's a consumable and I would keep one or two in case we want to transfer one rune and not another on the same weapon (because the target weapon can't handle it).