What is the difference between a rock and a mineral?
A mineral is a single natural substance with a fixed chemical makeup and crystal structure. A rock is usually a mixture of one or more minerals, and sometimes other materials.
Think of minerals as ingredients and rocks as the recipe. Granite, for example, is a rock made of several minerals, mainly quartz, feldspar and mica, which you can often see as different-colored grains. Some rocks, like limestone, are made mostly of one mineral, calcite. Others, like coal and obsidian, are not made of minerals at all.
Rocks are classified by how they form. Igneous rocks, like granite and basalt, form when magma or lava cools. Sedimentary rocks, like sandstone and limestone, form from layers of sediment pressed together. Metamorphic rocks, like marble and slate, form when heat and pressure change existing rocks.
The rock cycle
Over millions of years, rocks change from one type to another through melting, erosion, burial and pressure. This continuous process is called the rock cycle. It explains why the same material can be part of a mountain, then sand on a beach, then rock again.
Did You Know?
Marble is limestone that has been transformed by heat and pressure, and it is prized for sculpture because it is relatively soft and easy to carve.