Formation of Peridot
Would you believe me if I told you that there are volcanoes that erupt gemstones? It’s true!
Peridot is the gemstone name given to green olivine. Olivine crystals form in the Earth’s upper mantle at depths up to 250 miles, and they are brought to the surface by mafic (iron- and magnesium-rich) magma. Some of these crystals erupt with the lava at volcanoes in places like Hawaii.
Olivine is also formed in metamorphic rocks, near volcanoes. Olivine crystals can form in rocks called skarn – limestone that is transformed by fluids, heat, and pressure adjacent to hot intrusive magma.
Pictured to the right is Pu’u O’o, a volcanic vent on Kilauea volcano in Hawaii. This is the type of volcano that brings up olivine from the earth’s upper mantle.
Peridot has long been one of my favorite stones. I love its green color, its affordability, and the fact that it is one of the few gemstones mined in North America. I’m always a big fan of anything that can be sourced locally.
Olivine occurs in a range of green and brown shades, and some is even an amber brown color. The term peridot is reserved only for the green varieties. Peridot varies in color from lemon-lime to apple, to olive, and brownish green. The color doesn’t appeal to everyone, but it has always reminded me of spring. I love it.
Peridot has a Mohs hardness of 6.5-7, so it can be scratched by quartz. It is a bit on the brittle side too, so jewelers need to make sure to set peridot gemstones in a way that protects them from damage. Luckily for us jewelers, it’s fairly inexpensive, so it’s not entirely nerve-wracking to set.
Yellow-green peridot is the most highly prized hue. Stones over 5 carats are also highly valued because large olivine crystals are relatively rare. Myanmar is the only place in the world that large peridot specimens are routinely found.
Rock made of olivine grains (peridotite)
Olivine is a fairly common mineral, and it is found all over the world. Historically, Egypt was a great source of peridot gemstones. In the present day, peridot used in jewelry comes from places such as Arizona, New Mexico, China, Pakistan, and Myanmar.
Peridot is rarely enhanced to improve its appearance or durability. When you buy peridot, you can be fairly certain that it is natural and untreated.
Synthetics & Simulants
Because olivine is a fairly common mineral, it is not cost-effective to produce it synthetically in a lab. Green glass and crystal are often sold as imitations however.
Occasionally, peridot will exhibit a cat’s eye or asterism (star). These specimens are cut en cabochon to display the optical effect to its best advantage. I personally would LOVE to own a star or cat’s eye peridot.
Geology Nerd Note
You may be wondering how geologists know what depth olivine forms beneath the Earth’s surface. It’s actually very cool. Based on laboratory experiments, scientists are able to determine under what heat and pressure conditions certain minerals are stable. They have equipment that puts samples under extreme temperature and pressure conditions, and they measure the results. It turns out that olivine is stable only up to depths of about 250 miles beneath the Earth’s surface. The maximum depth at which olivine can form is therefore about 250 miles. To be clear, olivine also forms at shallower depths in the Earth’s crust.