Carbonation Stone, a device used to diffuse carbon dioxide into beer. It may be made of naturally porous stone, porous ceramic, or finely sintered stainless steel. Usually used inside a brewery bright tank or a brewpub serving tank, the carbonation stone is a hollow cylinder, capped at one end, into which carbon dioxide is forced under pressure. The CO2 diffuses through the stone, emerging on the exterior in very small bubbles. Under pressure, the small bubbles of CO2 dissolve into the beer before they can reach the surface. Carbonation stones may also be used in-line. A carbonation stone can be used to carbonate flat beer, to add CO2 to beer with inadequate carbonation for packaging or service, or to scrub dissolved oxygen from beer or water. Whether made of stone, ceramic, or sintered steel, the devices are colloquially known as “stones.”