Species and chemistry
Peridot is the gem variety of the forsterite-fayalite series — magnesium iron silicate (Mg,Fe)₂SiO₄. Most gem-grade peridot is forsterite-rich with a small fayalite component; the iron content is responsible for the characteristic green color.
Mohs hardness is 6.5 to 7; specific gravity is 3.27 to 3.48. Peridot is biaxial and positive.
Sources
- Pakistan — Sapat valley, Kohistan: source of large, saturated peridot crystals since the 1990s.
- Arizona, USA — San Carlos Apache reservation: long-established source of small to medium gem material.
- Myanmar — Mogok region: historic source of saturated peridot in larger sizes.
- China — Jilin and Hebei provinces: commercial-grade material.
- Egypt — Zabargad Island in the Red Sea: ancient source, with very limited current production.
Treatments
Peridot is generally untreated. The species is not amenable to heat treatment for color enhancement, and synthetic peridot is not commercially common. Authentication of unusual color or clarity should be performed by a gemmological laboratory when warranted.
Lapidary considerations
Peridot's distinctive feature in the workshop is its strong birefringence: at 0.036, doubling of pavilion facet edges is visible to the naked eye in larger stones. Cutters orient the preform along the optic axis to minimize the effect; some published cut designs are modified specifically for peridot's high birefringence.
The species is moderately heat-sensitive. Cold-cure epoxy is sometimes preferred over hot-melt wax for dopping larger Pakistani crystals.
Final polish is conventionally performed with cerium oxide or fine diamond paste; the species takes a clean optical finish.