How Do You Tell the Difference Between a Mineral and a Rock?

Crystal Structure

• All minerals have an ordered, crystalline molecular structure, and specific types of minerals always form the same shapes of crystals. If crystals are visible, you are looking at a mineral. Some mineral crystals are too small to be seen without a microscope or x-ray diffraction.

Chemical Composition

• Unlike rocks, each mineral has a very specific chemical formula, meaning that each molecule of the mineral contains the same atoms in the same arrangements. A rock can contain many different minerals and therefore have no consistent chemistry throughout. The only exception are rocks that are made up of only one mineral; for instance, sandstone is a rock mostly made up of the mineral calcite.

Rarity

• There are many different types of rock, but far more individual minerals. Most rocks are made up of a mixture of just a handful of the most common minerals, such as quartz, feldspar, mica and calcite. Over 4,000 different minerals exist, most of which are incredibly rare.