Broadly speaking, there are three kinds of rocks. They are: sedimentary (e.g. sandstone, limestone, slate and shale), igneous (e.g. granite, basalt) and metamorphic (e.g. marble, gneiss, schist). Sedimentary rocks are typically formed in the presence of water. Limestone often has identifiable organisms (shells, skeletons). Igneous rocks are formed from cooling of molten rock. Metamorphic rocks are formed when rock is transformed under heat and pressure. For example, when limestone can be transformed to marble.