A mineral must be inorganic and a solid. To be inorganic, a substance cannot be made up of living things or the remnants of living things; thus, many carbon-based substances, such as coal, are not minerals because the carbon comes from dead organisms.
A solid is one of the four states of matter (the other three are liquid, gas and plasma), and its primary characteristics are a consistent structure and a high resistance to changing shape and volume.
All minerals have a crystalline structure, which means their atoms are arranged in an orderly, three-dimensional manner, with this pattern repeating itself all the way throughout the mineral. Thus, there is no variation in structure at any point within the shape of the mineral; this is also known as being "physically homogeneous."
Additionally, all atoms in a mineral must be naturally occurring; synthetically (meaning man-made) produced compounds are not minerals.
While all minerals have certain properties in common, this does not mean there is not a wide variation within their other physical properties. Some physical properties by which minerals can be judged include their hardness, which is how much they resist being scratched, and is measured by the Mohs Hardness Scale; luster, which is an evaluation of how much light they reflect; and their cleavage, which describes how easily a mineral crumbles apart.
There are thousands of known minerals, though only a handful are considered common. The softest mineral on the Mohs scale is talc, while the hardest is diamond (which is a carbon-based mineral, but the carbon does not come from organic substances).
Minerals also can be classified based on the elements within them. For example, there is the oxide group, which is made up of minerals that have oxygen in them (such as magnetite), and the halides, which are minerals that have either fluorine, chlorine, bromine or iodine in them (such as common table salt, which has chlorine combined with sodium).