Slate is a very stable natural material.  It is thermally stable and not readily changed by chemical or biological reaction.  Slate comes in various colors, usually in shades of pale grey to dark grey, although it may also be purple, green or cyan depending on where it's mined.  Slate is quarried all over the world, either by open-pit mining or by tunneling. In the U.S., slate is extracted from eastern Pennsylvania, Virginia, New York, Vermont and Main. This fine grained material was originally composed of clay or volcanic ash in prehistoric ocean beds.  Through millions of years of pressure and extreme heat, known as metamorphism, it turned into what is known as slate. Because of the natural way in which slate can be split along its planer axis and yet still maintain its integrity, slate has been adapted very well to the building industry.