text: Minerals are natural compounds formed through geological processes. The term "mineral" encompasses not only the material's chemical composition, but also the mineral's structure. Minerals range in composition from pure elements and simple salts to very complex silicates with thousands of known forms (organic compounds are excluded). The study of minerals is called mineralogy. To be classified as a "true" mineral, a substance must be a solid and have a crystal structure. It must also be an inorganic, naturally-occurring, homogeneous substance with a defined chemical composition.\n\n--------------------------------------------------------------------------------\n\nNatural resources are naturally occurring substances that are considered valuable in their relatively unmodified (natural) form. A commodity is generally considered a natural resource when the primary activities associated with it are extraction and purification, as opposed to creation. Thus, mining, petroleum extraction, fishing, and forestry are generally considered natural-resource industries, while agriculture is not. Natural resources are often classified into renewable and non-renewable resources.