Peridotite is a coarse grained igneous rock which forms in the earth's mantle. It is the dominant and most common rock of the earth's mantle. Its high concentrations of Olivine and pyroxene contribute to its unique characteristics. The Olivine gives it a high magnesium concentration as well as an generous amount of iron. Peridotite itself belongs to the Amphibole group. It contains less then 54% silica making it an ultramafic rock, and therefore is made up of dark minerals with intense concentrations of magnesium and iron. There can be different Chemical Compositions with different Ratio's of Iron to magnesium, or Fe/Mg.