is a hard sedimentary rock
, composed mainly of calcium carbonate or dolomite, used as building material and in the making of cement. Limestone is generally white, but may exhibit a number of hues including blue, brown, black, grey, red, or yellow. These colors can be caused by impurities such as iron oxide and carbon. Though many limestones are alike in chemical make-up, the intricate basis of carbonate sediments results in an extreme diversity of substances composing these types of sedimentary rock.