Porous rock is a type of rock that has many small holes and cavities, allowing water and other liquids to pass through it. Two common types of porous rock are tufa and tuff. Tufa is a type of limestone formed by the precipitation of calcium carbonate from water, usually in a freshwater lake or river. Tuff is a type of rock formed from volcanic ash that has been compacted and cemented together. Both tufa and tuff are highly porous rocks that are often used in construction and landscaping.