Is olivine the same as peridot
Peridot is gem-quality magnesium-rich olivine. Olivine itself is a series name, spanning forsterite at density 3.275 and fayalite at 4.392, a 34.1% spread.
Peridot is the gem trade name for gem-quality, magnesium-rich olivine. Olivine is not a mineral species either: it is a series running from forsterite, Mg2SiO4, at density 3.275 and hardness 7, to fayalite, Fe2SiO4, at 4.392. That is a 34.1% density spread inside one word, and neither name appears on the IMA list.
In short
- Neither word is a species. The September 2026 IMA list has no entry for olivine and none for peridot. The valid species are forsterite, the magnesium end, and fayalite, the iron end, with a complete solid solution between them. Olivine names the series; peridot names a quality and a colour within it.
- The word olivine spans a 34.1% density range. Forsterite is 3.275 and fayalite is 4.392. Refractive index runs from 1.635 to 1.670 at the magnesium end and 1.827 to 1.879 at the iron end. A label that says only olivine therefore tells you very little about the physical properties of the thing in the box.
- Gem peridot sits near the magnesium end and barely varies. That is why peridot has a usable gemmological signature at all: the colour range is narrow, the yellowish-green is consistent, and it is one of very few gems that occurs in essentially one colour.
- Forsterite is named after an English mineral dealer. The Handbook records the name as honouring Adolarius Jacob Forster, 1739 to 1806, mineral collector and dealer. Fayalite is named for Faial Island in the Azores — where, the same source notes, the original material was probably slag from ship's ballast rather than a local rock.
- Most peridot a collector sees is from four places: Peridot Mesa at San Carlos in Arizona, the Mogok district in Myanmar, Sappat in Kohistan, Pakistan, and Zabargad in the Red Sea. Pallasite meteorites are the fifth, and the only source that is not terrestrial.
| Name | What it is | Status | Hardness | Density | Refractive index |
|---|---|---|---|---|---|
| Peridot | Gem-quality forsteritic olivine | Trade name; not on the IMA list | 6.5 to 7 | About 3.34 for gem material | Near the forsterite end |
| Olivine | The forsterite-fayalite series | Series name; not on the IMA list | 6.5 to 7 | 3.275 to 4.392 | 1.635 to 1.879 across the series |
| Forsterite | Mg2SiO4, the magnesium end member | Valid species, grandfathered 1824, Italy | 7 | 3.275 | 1.635 to 1.670 |
| Fayalite | Fe2SiO4, the iron end member | Valid species, grandfathered 1840, Portugal | 6.5 to 7 | 4.392 | 1.827 to 1.879 |
| Chrysolite | An obsolete name for the same material | Abandoned; avoid | — | — | — |
One mineral name, two species, and a third of a density between them
The Handbook of Mineralogy files both end members under olivine group and records each as forming a series with the other. Their properties are not close.
Forsterite: Mg2SiO4, orthorhombic, hardness 7, measured density 3.275 against a calculated 3.271, refractive indices 1.635, 1.651 and 1.670, colour green to lemon-yellow. Fayalite: Fe2SiO4, the same system and the same imperfect {010} and {100} cleavages, hardness 6.5 to 7, measured density 4.392, refractive indices 1.827, 1.869 and 1.879, colour greenish yellow to brown.
The density difference is 34.1%, and the refractive index difference is larger in proportional terms than the gap between quartz and topaz. Both are called olivine. A single word covering that much variation is doing the job of a mineral group, and the IMA list agrees: neither olivine nor peridot appears on it.
This is the same pattern as lepidolite, tetrahedrite, apophyllite and heulandite: a name coined for an appearance, later found to cover a range, and superseded by suffixed or end-member species names that nobody uses in conversation. Five of the 73 species pages on this site carry such a name, and each says so on the page.
What peridot fixes that olivine does not
Peridot is a gemmological term, and gemmological terms carry quality as well as composition. GIA's peridot description describes it as the gem variety of olivine, occurring in a narrow range of yellowish green to greenish yellow, and notes that it is one of the few gems found in only one colour.
That single-colour fact is a compositional statement in disguise. Gem peridot is magnesium-rich olivine with a modest iron content — iron is what produces the green — and material far from that composition is either too pale, too brown or too opaque to cut. So peridot effectively fixes the point on the series that olivine leaves open, and pins density near 3.34 and refractive index near the forsterite end.
Two practical consequences. First, a peridot's density and optical constants are predictable in a way an olivine's are not, which is why gemmologists can work with the term at all. Second, a mineral specimen labelled peridot is being described by its gem quality rather than its mineralogy, and the correct specimen label is forsterite with the locality, or olivine (forsterite-fayalite series) if no analysis exists. Label practice generally is on how to label and catalogue a mineral collection.
Where it comes from, and what it costs to own a good one
The Handbook's forsterite localities for near-end-member material or good crystals include Monte Somma and Vesuvius in Italy; Ojamo in Finland; the Kovdor massif on the Kola Peninsula and Zlatoust in the Urals; the Eifel district in Germany; Peridot Mesa, four kilometres southwest of San Carlos in Gila County, Arizona; and the Mogok district in Myanmar.
Fayalite's are quite different, because iron-rich olivine forms in iron-rich and silica-rich rocks rather than in mantle material: the St Peters Dome area near Pikes Peak in Colorado, Obsidian Cliff in Yellowstone, Rockport in Massachusetts, the Brocken massif in the Harz, and the Skaergaard intrusion. The two ends of one series come from opposite ends of igneous petrology, which is the clearest possible demonstration that the word olivine is describing chemistry rather than geology.
The honest position on cost, which is the thing nobody states. Faceted peridot is inexpensive as gemstones go and always has been, because the material is abundant. Crystallised specimen olivine is the opposite: sharp, gemmy, undamaged crystals on matrix are scarce, and the Pakistani and Myanmar material that supplies them passes through a long chain. A collector buying specimen-grade olivine is paying for crystal form and locality, not for the mineral, and a cut stone and a crystal of identical weight are not remotely comparable purchases.
Meteoritic olivine is its own case: pallasites carry gem-quality olivine in a nickel-iron matrix, and that is a meteorite question rather than a mineral one — see how to tell if your rock is a meteorite.
The rest of this section is indexed at the collecting guides. We hold no catalogue and nothing on this site is offered for sale; the wanted list sets out the localities and the crystal quality we look for.