The fundamental distinction between limestone and marble is that limestone is a sedimentary rock made up of of calcium carbonate fossils, whereas marble is a metamorphic rock made up mostly of calcium carbonate fossils. Marble is a metamorphic rock, meaning it has undergone a transformation or metamorphosis. This occurs when natural heat and pressure inside or on the surface of the Earth alters a rock over thousands of years. Formed when limestone (another rock) is subjected to extreme heat and pressure. It's largely made up of calcite, with pyrite, quartz, iron oxide, and granite thrown in for good measure. Marble is meant to deteriorate over time due to acid rain and weathering.