Peridot can be found in mafic and ultramafic rocks occurring in lava and peridotite xenoliths of the mantle. The gem occurs in silica-deficient rocks such as volcanic basalt and pallasitic meteorites. Peridot is one of only two gems observed to be formed not in Earth's crust, but in the molten rock of the upper mantle.[citation needed] Gem-quality peridot is rare on Earth's surface due to its susceptibility to weathering during its movement from deep within the mantle to the surface.[citation needed] Peridot has a chemical formula of (Mg, Fe)2SiO4. Peridot is one of the birthstones for the month of August.