These metamorphic stones (geological name Gneiss) were formed from old sedimentary layers, mainly sandstone and some interbedded clays. During the metamorphic process (about 2,500 million years ago) in India the stones were subject to heating and very high pressures. Water is expulsed together with many soluble chemicals such as iron. During this process new minerals are formed mainly muscovite, which gave the stone its layered structure and colour. The high grade metamorphic stones such as Silver Grey do no longer contain soluble elements, which were eliminated during their formation. As a result this stone has an extreme low porosity and permeability, it doesn’t have any free elements such as iron. For these reasons it is very well suited in salt and chlorine pools and for cladding exposed to air-pollution.