The group
Peridot is idiochromatic: the element that colours it is a structural part of the mineral, not a trace contaminant. That is why it comes in one colour only, and why heat treatment is pointless — there is nothing to remove.
Most of it forms deep in the mantle and arrives at the surface in basalt. Some arrives from further away: peridot is found in pallasite meteorites, and gem-quality extraterrestrial peridot has been faceted. The historic source was Zabargad, an island in the Red Sea worked for three and a half thousand years; the commercial sources today are San Carlos in Arizona, Sapat in Pakistan and Myanmar.
Peridot has high birefringence, 0.036, which is visible: look through the table of a larger stone and the back facets appear doubled. That is diagnostic, and it is not a fault.
At 6.5 to 7 with imperfect cleavage it is reasonably durable, but it is sensitive to acid and to sudden temperature change. Never steam-clean it.
Every olivine compared
The whole group side by side. Where the constants overlap, the species cannot be named from a refractometer alone — which is exactly why the trade names mislead.
| Variety | Species | Colour | Mohs | RI | SG | Guide price |
|---|---|---|---|---|---|---|
| Fayalite | Olivine group | yellow | 6.5–7 | 1.827–1.879 | 4.39 | Faceted stones 60–300 USD/ct |
| Forsterite | Olivine group | green | 6.5–7 | 1.635–1.671 | 3.23–3.26 | Near-colourless faceted material 30–150 USD/ct |
| Olivine (Foundry and Industrial) | Crushed dunite - a near-monomineralic olivine rock - used as foundry moulding sand, slag conditioner and refractory, and increasingly for carbon mineralisation | green | 6.5-7 | 1.64-1.69 | 3.20-3.40 | $60-200 per tonne depending on grade and freight |
| Pallasite | A stony-iron meteorite: gem olivine crystals suspended in a nickel-iron matrix, thought to form at the core-mantle boundary of a differentiated asteroid | green | 4-5 metal, 6.5-7 olivine | 1.65-1.69 (olivine) | 4.50-4.90 | $15-60 per gram for Seymchan slices; $50-300 per gram for gem-grade Esquel and Imilac; jewellery-grade translucent olivine higher |
| Peridot | Olivine (forsterite–fayalite) | green | 6.5–7 | 1.654–1.690 | 3.27–3.48 | $20–400/ct |
Questions
Why is peridot always green?
Because iron is part of olivine's chemistry rather than a trace impurity. The colour is structural, which is also why peridot cannot be heat-treated into another colour.
Is peridot treated?
Normally not. There is nothing to improve by heating, and the colour is inherent. Fracture filling exists on heavily included material but is uncommon.
What is extraterrestrial peridot?
Gem-quality olivine recovered from pallasite meteorites. It is genuine peridot that formed outside the earth, it is faceted and sold as a curiosity, and it commands a substantial premium over terrestrial material.
Looking for olivine?
Tell us which variety, what size and what it is for. We source it, photograph it and quote it, with the locality named.
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