A mineral is a naturally occurring compound or element found in the Earth's crust. Unlike rocks which minerals build, a mineral has the same chemical composition throughout.

Unlike many of the compounds that we see in the laboratory, the formula of minerals is not quite so predictable. The formula can have a range of values and still be recognizable as a particular mineral. For example, the mineral Olivine, which is a pale green mineral can have a formula ranging from Mg₂SiO₄ to Fe₂SiO₄.