Rocks are comprised of minerals, but minerals are not comprised of rocks. Minerals are inorganic compounds with unique chemical structures and physical properties that occur naturally. Rocks are solid masses that don't naturally occur. For example, the minerals quartz and feldspar, when formed together, make granite, which is a rock.
While minerals are pure and made of the same substance, rocks are not. Some minerals have crystals and are pretty, but rocks do not form single crystals and are rarely pretty. While minerals have a definite shape and a single color, rocks do not have a single shape and are usually made up of different colors. While rocks can contain one or more fossils, minerals cannot. Geologists differentiate between rocks and minerals based on these differences and also based on rock- or mineral-specific tests.
Geologists rely on five tests to identify a mineral: color, shape, hardness, streak and luster. Minerals are measured using the Mohs hardness scale, which assigns numbers between 1 and 10 based on how hard or soft the substance is. Identifying a mineral by its shape is sometimes difficult, since some minerals, such as calcite, come in multiple shapes. Geologists have identified more than 3,000 unique minerals.
Rocks form in one of three ways. Igneous rocks form after magma or lava cools and hardens. Metamorphic rocks form deep beneath Earth's surface and are shaped by extreme pressure and heat. The third type of rock is the sedimentary rock, which can form from fossils when mineral-rich water evaporates and leaves behind material, or when pieces of different rocks compress together and form a new rock. Minerals do not need to form, because they already occur naturally in nature.