Peridot

Peridot, chemically olivine (Mg₂SiO₄), is an orthorhombic silicate that crystallizes in magnesium-rich environments, typically in basic and ultrabasic igneous rocks. Its characteristic green coloration derives from the presence of divalent iron (Fe²⁺) that partially substitutes magnesium in the crystal structure. Unlike many gemstones, peridot is not a product of deep metamorphism: it instead forms during the cooling of silica-poor magmas, or in exceptional cases is brought to the surface by volcanic eruptions involving the Earth's mantle. Its hardness of 6.5–7 on the Mohs scale makes it suitable for jewelry but requires caution in daily wear. Historically, peridot was extracted on the Egyptian island of Zabargad (Topaz) in the Red Sea as early as 1500 BCE, and the ancients frequently confused it with emerald. The most prized deposits today are found in Myanmar, Pakistan (Himalayas), and Arizona, where gemstone-quality peridot commands significant prices in the international market.

Peridot (olivine): Mg₂SiO₄ with possible isomorphous substitution Fe²⁺ ↔ Mg²⁺. Orthorhombic crystal system, space group Pbnm. Lattice parameters: a = 4.76 Å, b = 10.20 Å, c = 5.98 Å. Hardness 6.5–7 Mohs; density 3.27–3.37 g/cm³ (increases with iron content). Refractive index: nα = 1.635–1.680, nβ = 1.651–1.705, nγ = 1.670–1.723; birefringence Δn ≈ 0.035–0.043. Moderate optical dispersion (0.020). Weak pleochroism: yellowish-green to bluish-green tones. Spectroscopy: characteristic absorption at 1050 nm and band at 450 nm (Fe²⁺); Raman spectrum with dominant peaks at 820 cm⁻¹ and 970 cm⁻¹. UV fluorescence generally absent or very weak. Formation in ultramafic rocks (dunites, lherzolites) and olivine basalts; mineralogical associations include enstatite, diopside, magnetite. Gem deposits: Myanmar (Mogok Valley), Pakistan (Hunza Valley), Arizona (San Carlos), Egypt (Zabargad), Brazil, China. Treatments: rare; occasionally light heating to improve color. Moderate thermal stability; avoid thermal shock.