We have not sold a diffusion stone until now because we just could not find one at a decent price. Well, that has changed, and we can now offer one for less than twenty bucks. The stone fits 1/4" i.d. tubing, and can be fitted to the end of the gas-in dip tube inside a Cornelius style keg to force carbonate beer really fast. Or use it to build your own wort oxygenation system. Shown in picture attached to the wort oxygenation system, which is not included.