SciShow Rocks Box: Peridot

• Chemical Formula: Mg₂SiO₄
• Mineral Name: Forsterite of the Olivine Group
• Locality: Peridot Mesa, San Carlos Apache Reservation, Arizona

Peridot is the gemstone variety of olivine, a group of minerals that includes forsterite, fayalite, and others. Olivine can be found in volcanic rocks all around the world, because it’s usually carried to the surface by magma in the form of peridotite. Peridotite, a mixture of olivine, pyroxene, and other minerals, is the most common substance in the Earth’s upper mantle.

At Peridot Mesa, a volcano brought up chunks of olivine as xenoliths in basalt from the upper mantle a few hundred thousand years ago. Gem quality forsterite olivine has been hand mined from Peridot Mesa by the Apache for over one hundred years, and it’s the most productive peridot site in North America. This specimen was hand mined by a tribal member of the San Carlos reservation.