Limestone is a sedimentary rock formed from biologic precipitates or cemented seashell fragments, with whole marine fossils sometimes being visible. Its grain size varies from microscopic to very coarse. Composed of calcite (a calcium carbonate mineral), limestone forms many karst cavernous systems and makes soil calcium-rich. For example, horses in Ireland develop strong bones and teeth by grazing on grass that extracts calcium from the soil above limestone layers. Prominent limestone structures include Israel’s Temple Mount and the casing stones of the pyramids of Giza, Egypt.