English: Peridotite mantle xenolith in phonotephrite from the Pleistocene of Arizona, USA.

Green = peridotite Gray = phonotephrite host rock

“Peridot” is a gemological term for gem-quality forsterite olivine, but it does not differ in any chemical sense from ordinary olivine. Gem-quality olivine is known from several places on Earth, including Peridot Mesa in southeastern Arizona, USA.

The volcanic rocks making up Peridot Mesa are gray, slightly vesicular phonotephrites (formerly misidentified as basanites) of the Peridot Mesa Flow, a 3 to 35 meter thick lava flow erupted during the Middle Pleistocene (~580,000 years ago).

The Peridot Mesa lavas have common greenish-colored xenoliths derived from the mantle. The xenoliths are ultramafic, crystalline-textured, intrusive igneous rocks principally composed of olivine and/or pyroxene. Specific Peridot Mesa xenolith lithologies include spinel lherzolite, dunite, harzburgite, clinopyroxenite, websterite, and orthopyroxenite.

The green xenolith shown here appears to be spinel lherzolite or spinel dunite, dominated by lime green-colored, glassy-lustered forsterite olivine (Mg2SiO4). Other minerals include bright green chromian diopside pyroxene ((Ca,Cr)MgSi2O6), and black spinel (MgAl2O4).

Some of the green olivine crystals in Peridot Mesa mantle xenoliths are large enough, transparent enough, and richly green enough to be gem quality ("peridot").

Stratigraphy: Peridot Mesa Flow, middle Middle Pleistocene, ~580 ka

Locality: Peridot Mesa, San Carlos Volcanic Field, San Carlos Apache Reservation, southeastern margin of Gila County, southeastern Arizona, USA