This therapeutic treatment not only provides muscle massage but also aids in mental relaxation calming the central nervous system. The stones used for hot stone massage are basalt stones, a black volcanic rock, which contain a high iron content and superb capability to retain heat. They are heated in the Spa-Pro Massage Stone Heater until they are at the appropriate temperature. The stones vary in size, shapes and weights. Different stones are used for different parts of the body. Because of this Spa-Pro offers two different size stone heaters - 6 quart capacity and 18 quart capacity.