Types of Rocks
Rocks and stones can be defined as naturally occurring mixtures of minerals/organic compounds. Rocks and stones form the bulk of our Earth.
Sedimentary rocks are formed from particles of sand, pebbles, shells, and other fragments that are collectively termed sediment. In this category you have clays, limestones, and sandstone. The layers are normally parallel or nearly parallel to the Earth's surface; if they are at high angles to the surface or are twisted or broken, some kind of Earth movement has occurred since the rock was formed.
Igneous rocks are formed when molten rock (magma) cools and hardens within the Earth. In this category you have basalt, granite, obsidian, and volcanic ash. When magma cools slowly, usually at depths of thousands of feet, crystals grow from the molten liquid, and a coarse-grained rock forms. When magma cools rapidly, usually at or near the Earth's surface, the crystals are extremely small, and a fine-grained rock results. A wide variety of rocks are formed by different cooling rates and different chemical compositions of the original magma. Obsidian (volcanic glass), granite, basalt, and andesite porphyry are four of the many types of igneous rock.
Metamorphic rocks are formed under Earth's surface due to changes in heat and pressure. They may have ribbon-like layers of various colors in them. In this category you have things like marble and slate. The process of metamorphism does not melt the rocks, but instead transforms them into denser, more compact rocks. New minerals are created either by rearrangement of mineral components or by reactions with fluids that enter the rocks. Some kinds of metamorphic rocks--granite gneiss and biotite schist are two examples--are strongly banded or foliated. (Foliated means the parallel arrangement of certain mineral grains that gives the rock a striped appearance.)