Limestone is a type of hard sedimentary rock used in building materials. An interesting fact about limestone is when it is completely pure the colour is white. When limestone combines with other elements the colour changes to any colour in the rainbow. Limestone is calcareous rock in the widest sense. In Anglo-Saxon nomenclature this terms is often used to distinguish a non-crystalline marble (namely a calcareous stone = limestone) from a crystalline marble (metamorphic stone = marble).