A stone which mainly consists of calcium carbonate, sometimes with additions of iron, silica or clay. As a result there are different types of limestone of varying quality and hardness. The best quality limestone was mined in the quarries of Tura (near Cairo) and Gebelein, but limestone was in fact available almost everywhere: it is found in the hills alongside the Nile valley between Cairo and Esna. Limestone blocks could also be quarried right where they were needed, for example at Giza. The pyramids there are to a large extent made of limestone that was quarried from the plateau on which they stand. The Step Pyramid of Djoser at Saqqara is also partly made from limestone blocks quarried on site. After the middle of the 18th Dynasty, limestone began to be replaced as a building material by sandstone. Limestone was one of the first stone types to be used for making objects because it is relatively soft and easy to work.