All of these filters offer a porous material which sits in the base of the bowl, Silica is frowned upon by many smokers as unsafe and chalk is not overly popular. The Nording Keystones are made from Volcanic clay and offer a natural, tasteless and highly absorbent medium. This soaks up moisture from both the smoke and from the smoker, resulting in a cool and dry experience. These are ideal with the Erik Nording Eriksen Pipes, but can also be used with a conventional pipe too.