Olivine is a ferro-magnesian mineral that shows a complete compositional variation between forsterite (magnesium-rich) and fayalite (iron-rich). Olivine is light green in colour and glassy. The internal arrangment of the atoms is constructed of isolated silica tetrahedra (SiO₄) held together ionically by iron and magnesium ions. With no planes of weakness there is no cleavage, and because the crystals grow outward in all directions from a tetrahedral seed they are described as granular.

Olivine is a rock-forming mineral, and occurs in igneous rocks with low silica, such as a gabbro, basalt, or dolerite. This is a relatively rare mineral in most igneous rocks, or one that is hard to see without a microscope. Large accumulations form the igneous rock dunite or, when mixed with pyroxene, peridotite. Olivine is rare as a detrital mineral - on Earth* - owing to its low stability in sedimentary environments. Forsterite can occur in metamorphic marble. Fayalite is dark brown or black; it is uncommon but can occur in granite pegmatites.

*Olivine appears to be stable as a detrital mineral under the cold, dry conditions at the surface of Mars.