Nomenclature and classification are very important in science. Being very precise about what you are investigating helps scientists communicate findings and results across geographic and cultural boundaries. Non-scientists commonly use the terms “mineral” and “rock” interchangeably in everyday language, but in science they mean very different things. The International Mineralogical Association describes a mineral as an element or chemical compound that is normally crystalline. Crystalline refers to the orderly geometric spatial arrangement of atoms in the mineral. Table salt or sodium chloride (NaCl) is a familiar mineral that forms cube-shaped crystals. Typically the chemistry and crystalline structure of a mineral help scientists to classify it. Minerals are formed as a result of geological processes.

An example of a mineral is quartz. Quartz is made up of a crystalline structure of the compound silicon dioxide, SiO₂ (SF Fig. 7.10 A). By contrast, rocks are mixtures of minerals. The mixture may be homogenous or heterogeneous in nature. Homogeneous rocks are made up of more than one type of mineral, but are uniform throughout and usually one color. In heterogeneous rocks, the minerals are distributed unevenly, so they are often not uniform in color. Slate is an example of a homogeneous stone, with even mineral distribution and color. Granite is an example of a heterogeneous rock that contains a mixture of the minerals quartz, mica, and feldspar and is variable in color (SF Fig. 7.10 B).