The solid part of the earth's crust is rock. In general it is covered by a layer of soil in which plants or trees may grow. Even the soil is made up of tiny bits of rocks usually mixed with organic materials from plants and animals. Rock can also be found beneath the oceans and under the polar icecaps. Rocks and minerals are used in many ways. Builders use granite, marble, and other rocks in constriction work. Cement made from limestone and other rocks binds crushed stone into strong, long-lasting concrete for buildings, dams, and highways. Metals such as aluminum, iron, lead, and tin come from rocks that we call ores. Ores also supply such radioactive elements as radium and uranium. Nonmetallic minerals such as borax and graphite can also be found in rocks. All gems except amber, coral, and pearl, come from rocks. Diamonds mined in Africa and Arkansas come from a rock called peridotite. Emeralds are found in black limestone in Colombia. The history of the earth can be traced by studying rocks. Fossils, or imprints of plants and animals found in rocks, help scientists learn about the kind of life that existed millions of years ago. The three main kinds of rocks are: igneous rocks, sedimentary rocks, and metamorphic rocks. Igneous rocks form when molten or melted rock material called magma, cools and solidifies. There are two types of igneous rocks: extrusive rocks and intrusive rocks. Extrusive rocks form when magma is extruded, or forced out, onto the surface of the earth. This may occur when a volcano erupts great masses of hot lava, cinders, and ash. Lava is the name for magma that reaches the surface. Intrusive rocks form from magma that does not rise all the way to the surface of the earth. It may push up the surface rock in the shape of a huge blister. Beneath the surface, the molten rock cools and hardens slowly. Sedimentary rock consists of materials that once were part of older rocks or of plants and animals. These rocks were deposited millions of years ago as layers of loose material. As time passed, the loose materials changed into solid rocks. Metamorphic rock is rock that has changed its appearance and sometimes its mineral composition. These changes may be caused by hot magma, pressure and heat from mountain-building movements in the earth, or the chemical action of liquids and gases.