A mineral is any naturally occurring inorganic solid created by a geological process that has a specific chemical composition, a highly ordered atomic structure (usually possessing a crystal structure) and specific physical properties. Crystal structure is critical to a mineral’s physical properties. Diamond, the hardest naturally occurring substance, is made up of pure carbon, but its crystal structure is what separates it from graphite, a very soft substance that is also pure carbon.

Minerals can be pure elements or simple salts ranging to very complex silicates. Most inorganic gemstones are minerals (a very common one is quartz.) When minerals occur in an aggregate they form a rock. Most gemstones are not rocks, with a few notable exceptions such as lapis lazuli.

Minerals are classified by their physical properties. Some of these properties are easily observable and others require specialized equipment and extensive testing.