Rocks come in lots of different sizes, shapes and colors but there are only three rock types in the world. Rocks are either igneous, sedimentary or metamorphic based on the way they are formed.

Igneous Rocks
Igneous rock is formed when magma, also called molten rock, undergoes a change of phase and solidifies. Igneous rock makes up most of the Earth's crust but very little of it is visible. Most magma cools to form igneous rock far below the surface of the Earth and never finds its way to the top. When it does it is called lava and it erupts from a volcano. This detail may seem unimportant but it makes a big difference in the rock's texture. Magma cooled at the surface is fine-grained and smooth because it happens quickly. Magma cooled underground is coarse and has large crystals because it happens slowly. Granite, basalt and obsidian are examples of igneous rock.

Sedimentary Rocks
Sedimentary rock is formed when deposits such as sand, shells and pebbles are pressed together from the weight of each new layer and cemented into stone by minerals. Sedimentary rock may not be the most abundant rock on earth but it is the type of rock you are most likely to find. And because sedimentary rock covers most of the Earth's surface it tells a great deal about the history of the land. The composition and texture tells us about the origin of the rock. Its contents, such as fossils, tell us about the environment in which it was made. Sandstone, limestone and conglomerate are examples of sedimentary rock.

Metamorphic Rocks
Metamorphic rocks, as the name suggests, undergo a change. Unlike caterpillars, metamorphic rocks don't sprout wings. And they don't change phases and melt. Igneous rocks do that. Instead, metamorphic rocks are formed when intense heat and pressure and some mineral-rich liquids cause the rock to breakdown and recompose in a new structure. Metamorphic rocks are easily identified by the folds and crystals that often form. And while metamorphic rock might not tell us about life on Earth it tells us a great deal about what happened below the surface. Marble, jade and slate are examples of metamorphic rock.