Tufa stone is a type of limestone which forms in close proximity to bodies of water with a high content of dissolved minerals, especially calcium carbonate. There are a number of uses for tufa, ranging from construction to artwork, and it is also interesting to view in situ. Tufa, incidentally, is not the same thing as tuff, a form of rock formed from volcanic ash, although the two rocks have many similar uses.
Some famous examples of tufa can be found at Mono Lake, a lake in California which is famous for its high concentration of dissolved minerals. Over time, these minerals have precipitated out, creating distinctive tufa towers which loom over the lake. Tufa can also be found around springs, former waterways, and other lakes with lots of dissolved minerals, and the shape of tufa stone formations is often quite fanciful, thanks to the processes of erosion and deposition.
A number of factors can contribute to the formation of tufa. In many cases, minerals build up along a shoreline as a result of repeated depositions from waves. Tufa stone can also form when extremely hot water from a spring enters a cooler body of water, encouraging the dissolved minerals from the spring to precipitate out, and tufa formations commonly appear as bodies of water dry up, move, or recede.
This stone tends to be soft and extremely porous. The porous nature of tufa makes it very popular for planters, as it will easily drain, rather than trapping water. Tufa can also be used as a sculpture medium, and it is used by some jewelers as a casting medium. To cast with tufa stone, jewelers and other metalworkers carve the desired shape into the rock, commonly forming a two-piece mold, and then pour hot metal into the mold. Although tufa is soft, it can handle high heat, so it will not crack or distort when used as a mold.
In areas with large natural deposits of tufa, the stone has also historically been used as a building material. Some constructions made with tufa stone are quite ancient, especially in desert regions, where the arid conditions have prevented erosion of the tufa and the mortar used to cement it together. Tufa constructions are also sometimes decorated with interesting carvings and inlays, some of which manage to endure the test of time.
Rock shops and some art supply stores carry tufa stone in a variety of sizes, and it can also sometimes be purchased through construction companies.