These metamorphic stones, known geologically as Gneiss, were formed from ancient sedimentary layers, mainly sandstone and some interbedded clays. During the metamorphic process, approximately 2,500 million years ago in India, the stones were subjected to intense heating and very high pressures. Water, along with many soluble chemicals such as iron, was expelled. New minerals, primarily muscovite, formed during this process, giving the stone its layered structure and color. High-grade metamorphic stones like Silver Grey no longer contain soluble elements, which were eliminated during their formation. As a result, this stone has extremely low porosity and permeability and does not contain free elements such as iron. These properties make it very well suited for use in salt and chlorine pools and for cladding exposed to air pollution.