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Peridot

Peridot is gem-quality olivine colored in green color of different intensity, most often with a yellowish hue. Peridot is the fayalitic minal variety of olivine according to mineralogical classification with the "forsterite-fayalite" within the range of 10-30% (olivine itself in foreign literature). Peridot is the gem variety of the mineral olivine in English-speaking countries. This name was registered at the CIBJO congress in 1968. The name "peridot" comes from the ancient Greek - "χρυσός" - "gold" and "λίθος" - "stone" (golden stone).

Chemistry: (Mg,Fe)2[SiO4];

Crystal system: Rhombic, Rhombo-dipyramidal crystal class (point symmetry group);

Color:  Peridot crustal structure includes iron and iron Fe2+ is the cause of its attractive yellowish green colors, and yellow and brown colors are associated with an isomorphic admixture of iron Fe3+. Also peridot may vary in colors due to the isomorphous impurities Cr3+ and/or V3+.

Identification properties

Physical properties
Mohs hardness: 6.5 – 7
Density: 3.28 – 3.48 g/cm3
Cleavage: imperfect to medium on (010), imperfect on (100)
Fracture: uneven, conchoidal
Optical properties
Optical character: anisotropic, biaxial (from negative to positive)
Refractive Index: np =1.650 – 1.693, nm =1.671 – 1.716, ng =1.686 – 1.731
Birefringence: 0.036 – 0.038
Pleochroism: poor
Dispertion: 0.020(BG)
Luster: vitreous, glassy

Inclusions and structural inhomogeneities