Peridot is the gem-quality form of olivine, a magnesium iron silicate that is one of the most common minerals in the Earth's upper mantle. Its yellowish green to pure green color comes from iron that is part of the mineral's own chemistry, so peridot is only ever found in shades of green. It is the modern birthstone for August, and some peridot has even arrived on Earth inside meteorites.
What peridot is
Olivine is a solid solution series between forsterite (Mg2SiO4) and fayalite (Fe2SiO4). Gem peridot sits close to the forsterite end, with roughly 10 to 15 percent iron. That iron is what colors it. Because the coloring element is part of the mineral's own formula, peridot is called idiochromatic, or "self-colored." More iron gives a deeper, browner green; less gives a lighter, yellower green. The color in gemstones page explains idiochromatic and allochromatic color.
Peridot crystallizes in the orthorhombic system. It has a Mohs hardness of 6.5 to 7, a refractive index of about 1.654 to 1.690, and a specific gravity around 3.3 to 3.4. Its birefringence is high, about 0.036, which means that when you look through the table of a faceted stone with a loupe, the back facet edges appear doubled. This doubling is one of the easiest ways to identify peridot, and the birefringence and double refraction page explains why it happens.
Typical inclusions include small dark chromite or spinel crystals surrounded by flat, disc-shaped stress fractures. Gemologists call these "lily pad" inclusions, and they are a classic sign of natural peridot.
How peridot forms
Most olivine forms deep in the Earth. Peridot reaches the surface in two main ways:
- Mantle rock fragments, called xenoliths, are carried up in basalt lava. These peridotite nodules break apart to release peridot grains. Arizona and China produce peridot this way.
- Slices of mantle rock pushed up onto continents during mountain building, sometimes altered by later fluids, can host larger crystals in veins and pockets. The deposits in Pakistan, Myanmar and on Zabargad island formed in such settings.
Peridot is also one of the few gems that forms in rocks from space. Pallasite meteorites, a type of stony-iron meteorite, contain olivine crystals set in iron-nickel metal. Rarely, these crystals are clear enough to facet. The igneous rocks and gemstones page covers basalt-hosted gems in more detail.
Zabargad island and ancient peridot
The oldest recorded source is Zabargad (also called St. John's Island), a small island in the Red Sea off the coast of Egypt. Ancient Egyptians mined peridot there, and the island was difficult to find in fog, which added to its reputation. The ancient Greek and Roman writers called the island Topazios, and the gem they called "topazos" was very likely peridot, not the mineral we now call topaz.
Zabargad peridot was traded through the Mediterranean for centuries. Large peridots set in medieval European church treasures, including those in the shrine of the Three Kings at Cologne Cathedral, were long thought to be emeralds. Many of these stones probably came from Zabargad. The deposits were rediscovered and worked again in the early twentieth century and produced some of the finest large peridots known before production stopped.
Modern sources
Arizona and the San Carlos Apache Reservation
A large share of commercial peridot has come from the San Carlos Apache Reservation in Arizona, where peridot grains weather out of basalt. Apache miners dig and sort the material. Most San Carlos stones are small, under a few carats, with a yellowish to medium green color. The main digging area there is Peridot Mesa, and peridot is also found in New Mexico.
Pakistan
In the 1990s, high mountain deposits in the Kohistan region of northern Pakistan began producing large, clean peridot with a rich green color. Stones over 10 carats became far easier to find than before, and Pakistani material is now associated with top color.
Myanmar and others
Myanmar has produced fine peridot from the Mogok region for many years, including large stones with a deep green. China (Jilin and Hebei), Vietnam, Ethiopia, Tanzania, Kenya and Norway are also sources. The rock cycle and gem formation page puts these deposit types in context.
Color and value
The most valued peridot color is a pure, saturated green with only a slight yellow tint. Strong yellow or brown tones lower value. Size matters a great deal: small stones are common, while clean stones over 10 carats with fine color are scarce and priced much higher per carat.
Peridot is almost never treated. Some stones may be filled with oil or resin to hide surface-reaching fractures, but this is uncommon. Glass and green synthetic spinel have been used as imitations, and the doubling of back facets is a quick way to separate peridot from single-refracting look-alikes. Synthetic peridot is not a commercial product.
Care and durability
Peridot is softer than quartz at 6.5 to 7, so its facet edges can wear and its surface can dull over years of daily contact with dust and hard surfaces. It also has poor cleavage and can be brittle. It is sensitive to acids, including hydrofluoric and strong hydrochloric acid, and should not be exposed to harsh chemicals.
Peridot is best set in rings with protective mountings and worn with some care, or used in earrings and pendants. Clean it with warm soapy water and a soft brush. Avoid ultrasonic and steam cleaners, and avoid sudden temperature changes. See the jewelry cleaning guide and gemstone durability for more.
History and birthstone
Peridot has been worn since antiquity in Egypt and the Mediterranean, and it was popular in Victorian and Edwardian jewelry. Some folklore linked it with the sun and with protection from night terrors. These ideas are traditional beliefs, and similar stories are collected on the gemstone meanings and folklore page.
Peridot has been an August birthstone since the 1912 American list, which also named sardonyx for the month. In 2016 spinel was added as a third August stone. See birthstones by month. Peridot is also a traditional gift for the sixteenth wedding anniversary in some lists.
Frequently asked questions
Why is peridot only green?
Iron is part of peridot's chemistry, so the stone is always some shade of green. The amount of iron controls whether it leans yellowish or deep green.
How can I identify peridot?
Look through the table with a loupe. Peridot's high birefringence makes the back facet edges appear doubled, and lily pad inclusions are common.
Is peridot from meteorites real peridot?
Yes. Olivine crystals in pallasite meteorites are the same mineral, and rare clear pieces have been cut as gems.
Is peridot treated?
Peridot is rarely treated. Occasionally surface fractures are filled with oil or resin, which should be disclosed.