If the stone is being stubborn, try soaking the entire pipe in ISO for a hour or so, to help loosen any residue. Drain and try tapping the stone out. OBVIOUSLY do not use any flame using this method. Make sure to fully let everything dry out completely before using. Once the stone is removed and dry, torch it until glowing orange and let cool down 15 min before using again.
Usually the initial method is the preferred method, but ISO will work and be ok, but requires more attention by the user.
Thank you!