A mineral is a naturally-occurring, inorganic solid substance with a crystalline structure, a fixed or variable chemical composition, and a set of physical properties that can be used for identification.
Smoky quartz, shown here, is an example of a mineral. It occurs naturally and is solid and inorganic. Its internal structure can be seen in its crystal form. Its chemical composition is SiO2, or a combination of silicon and oxygen. Its physical properties can be used to identify it. (Photograph by Parvinder Sethi)
Naturally-occurring means that minerals are formed through natural processes. Synthetic (human-made) materials are not minerals
Inorganic means that minerals are formed from non-living materials. Organic materials, such as fossils, oil, and gas, are not minerals.
This plant fossil was formed from living materials. Therefore, it is not a mineral. (Photograph by Parvinder Sethi)