Minerals are the building blocks of the earth. A mineral is a combination of elements that forms an inorganic, naturally occurring solid of a definite chemical structure. For example, SiO2 is always the mineral quartz. A rock is a solid material that is composed of various minerals.
Minerals can have a variety of crystalline shapes. The shape of the crystal is dependent on the sizes of the atoms of the elements, the chemical bonds that hold the elements together to form the mineral, and the pressure and temperature at which the mineral formed.
Most minerals are built around silica tetrahedrons—four oxygen atoms connected to a smaller, central silicon atom. Different arrangements of silica tetrahedrons create distinctive atomic structures in minerals, such as sheet silicates (the mica and clay mineral groups), chain silicates (the pyroxene mineral group), or framework silicates (the quartz and feldspar mineral groups).
Only several hundred of the thousands of known minerals are important rock‐forming minerals. As one might guess, their chemical compositions contain mostly the eight most common elements in the crust—oxygen, silicon, aluminum, iron, calcium, sodium, potassium, and magnesium. The important rock‐forming mineral groups are quartz, feldspars, amphiboles, pyroxenes, clays, micas, and carbonates.
A rock's color is determined by its mineral components: quartz, feldspars, carbonates, and some micas are generally light‐colored, tan, or pinkish; pyroxenes, amphiboles, and some micas are dark green to blackish because of their high iron and magnesium content.