What peridot actually is
Peridot is the gem-quality variety of olivine, a magnesium-iron silicate solid solution series with the formula (Mg,Fe)₂SiO₄[IMA][GIA]. The series spans two end-members: forsterite (Mg₂SiO₄, magnesium dominant) and fayalite (Fe₂SiO₄, iron dominant). Commercial peridot sits well toward the magnesium-rich forsterite end, typically Fo₈₀–₉₅, with iron concentration governing the depth of green color[USGS]. The species crystallizes in the orthorhombic system with vitreous luster, indistinct cleavage, conchoidal fracture, and the diagnostic high birefringence of 0.036. The latter is the cause of the "fuzzy" appearance peridot can show under poorly oriented cutting[IGS][Gem-A].
The optical fingerprints labs use to identify peridot are refractive index 1.654–1.690 (varying with iron content), specific gravity 3.27–3.48, dispersion 0.020, Mohs hardness 6.5–7, and biaxial optic sign[IGS]. The high birefringence is unusual among colored gems and contributes to the diagnostic doubled-back-facet appearance under loupe examination.
Olivine is one of the most abundant minerals in Earth's upper mantle, but the gem-grade fraction, large clean crystals of forsterite-rich composition, is geologically rare[USGS]. Peridot rough is recovered from volcanic xenoliths (where mantle olivine has been carried to the surface in ascending magma), from stratified metamorphic peridotites, and, uniquely, from extraterrestrial pallasite stony-iron meteorites[Lotus].
Color chemistry
Peridot's color is intrinsically yellow-green, with the precise hue determined by the balance of ferrous and ferric iron in the structural lattice[GIA]. Ferrous iron (Fe²⁺) drives the green. Ferric iron (Fe³⁺) shifts the color toward yellow. Higher overall iron content produces the brownish-green tones at the muddier end of the range[IGS]. Trace chromium contributes to brightening saturation in select material but is not the primary chromophore. Peridot is fundamentally an iron-driven green, distinguishing it from emerald (chromium-driven) and tsavorite garnet (chromium and vanadium-driven)[GIA].
The most prized peridot color is a richly saturated pure grass green, yellowish-green hue with medium tone and moderately strong saturation. This color tier is realized predominantly in stones over 10 carats. Smaller material tends toward more yellowish-green tones. The chromophore concentration is fixed by the rough's iron composition, but cutting orientation can amplify or mute the color through pleochroism (peridot shows weak pleochroism with yellow-green and yellow visible along different optical directions)[IGS][GIA].
Origin science
Peridot's geographic-origin landscape is tightly tied to the geological mode of formation[USGS]. Pakistani Naran-Kaghan and Mansehra deposits in the Hindu Kush produce the modern apex material, large, clean, transparent crystals from pegmatite-hosted peridotite, with the finest large peridots in the trade typically from this source[IGS]. Burmese Mogok produces historical premium material with comparable clarity and color saturation. Egypt's Zabargad Island in the Red Sea, the ancient Egyptian secret source, produces material that commands a slight premium today on historic-provenance grounds. Arizona's Peridot Mesa on the San Carlos Apache reservation produces the abundant commercial supply at smaller-stone sizes from basalt-hosted xenoliths[USGS].
Tanzanian, Vietnamese, Madagascar, and Chinese deposits round out the modern supply at varying quality tiers[Gem-A].
Treatment landscape
Peridot is essentially never heat-treated commercially, and the species is among the least-treated colored gems on the market[GIA][LMHC]. Heat treatment to alter color is not part of the trade vocabulary. The iron-driven chromophore does not respond to heat in a price-positive way. Occasional fracture filling with colorless oils, waxes, or resins to improve surface luster on heavily-included material is documented but uncommon. Any such enhancement is required to be disclosed per CIBJO[CIBJO]. Foiling on cut peridots to enhance stability and color is a niche treatment in older jewelry, not in modern trade.
Peridot is effectively always unheated, reflecting the actual commercial state of the trade.
Phenomena science and pallasite
Lily-pad inclusions are the diagnostic peridot inclusion pattern: disk-shaped reflective inclusions surrounding minute mineral crystals, formed during slow cooling of the host olivine in mantle-derived rock[Gem-A][GIA]. Lily pads appear as pale, circular reflective discs under transmitted light and are distinctive enough to identify peridot from natural-vs-synthetic comparison and to provide weak provenance signal across deposits.
Cat's-eye peridot from oriented inclusion needles is documented but rare per IGS. Cabochon cutting is required to display the chatoyancy[IGS].
Pallasite peridot is gem-quality olivine extracted from pallasite stony-iron meteorites, a class of stony-iron meteorites where olivine crystals are embedded in an iron-nickel matrix[Lotus][USGS]. Mineralogically the peridot itself is identical to terrestrial material (same forsterite-fayalite chemistry, same crystal structure), but the geological origin from outside Earth's mantle creates a collector premium independent of standard color-and-clarity grading. The Imilac, Esquel, and Brenham pallasites are the historically significant sources for cuttable extraterrestrial peridot, and the rough that survives the pallasite's terrestrial impact and weathering as gem-grade material is an extreme rarity[Gem-A]. Pallasite peridot commands a distinct phenomenon-grade premium.
Species identity synthesized from the authoritative sources cited below.