Mafic is a word that is used to define igneous rocks with a high iron and magnesium content. The olivine minerals have a high melting point and are the first minerals to crystallize from a mafic magma. Forsterite crystallizes first with fayalite crystallizing last when other minerals such as the pyroxenes are just beginning to form. The early crystallization of olivine is the reason that molten lavas can contain already crystallized grains of olivine. Some ultramafic rocks can be composed of almost all olivine and these are called dunites or peridotites .
Peridotites contain the same chemical makeup as the molten magma in the Earth's mantle.
Thus peridotite could be called the most common rock by volume in the Earth, although on the Earth's surface and in the crust it is not nearly as well represented. Olivine can be altered to the mineral serpentine and this mineral is found in kimberlites, ophiolites and gabbros that started out with large amounts of olivine at one time. Olivine is also found in many iron-nickel meteorites. Thinly cut slices of these meteorites are extremely attractive with the polished steel gray of the iron and the embedded grains of gemmy green olivine. The effect produces the closest mineral equilalent to stained glass artwork.