Limestone is a sedimentary rock composed largely of mineral calcite. Limestone is the result of millions of years of seashells and sea creatures piling up on top of each other. The calcium from the bones and shells combines with carbon dioxide and water to form calcium carbonate, the basic mineral structure of all limestones. Pure limestone is almost white. Various impurities in the stone like clay, sand, and iron oxide cause limestone to come in a variety of colours. A distinguishing characteristic of many limestones is the presence of fossils that are frequently visible in the surface of the stone. It is very reactive to acid solutions and can etch easily.