Slate
From Conservapedia
Slate is a metamorphic rock created by the application of intense pressure on mudstones and siltstones during periods of orogeny many millions of years ago. As the mud or silt was laid down in thin layers slate is a highly fissile rock that can be split into thin sheets.
Slate has found use as a roofing or flooring material, for the bed of a snooker or billiards table and for writing tablets. Slates for writing have been in use for many thousands of years and were common well into the twentieth century in many schools. Writing would be accomplished either with a coarse stick of chalk or a fine stylus made of graphite. Owing to the ease at which the slate tablets could be cleaned for re-use we get the English idiom to "start with a clean slate".
When slate undergoes further heat and pressure, it can metamorphose into schist.
