All rocks fall into the three main categories of igneous, metamorphic or sedimentary. These rocks vary widely in physical appearance and location and their formation patterns differ, as well: igneous rocks form cooling magma deep within the earth's core. Activity within the core produces metamorphic rock and accumulations of silt, sand and other earthen debris that form sedimentary rocks along the earth's surface.
Of the three main rock groups, igneous rocks reside deepest within the earth. These rocks require magma, a catalyst for growth. Magma produces igneous rocks as it cools and hardens. Common rocks in this category include granite, basalt and obsidian. Their appearance and physical characteristics vary, depending on the rate of magma cooling and presence of air. Rapid magma cooling leaves a smooth and shiny surface. Sometimes gas bubbles collect in cooling rock, creating small holes. Metamorphic rocks derive from movement of substances beneath the earth's surface. Changing pressures and temperatures create these rocks, which feature multiple layers and uneven surfaces. These rocks sometimes develop crystals and minerals; common examples include slate and marble. Sedimentary rocks form from accumulating deposits of shells and fossils. Over time, particles in these substances compress and harden, forming rocks. Rocks in this group include sandstone and limestone.