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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
Gem basic treatments
| Treatment | Goal |
|---|---|
| Heat treatment | Lightening or improving the green hue |
| Impregnation with various coloring compounds | Altering the gem visual color |
| Surface coating by applying a various coloring agent | Altering the gem visual color |
Synthetic or Imitation gem materials
Synthetic peridot for the jewelry industry is not produced. A wide application of synthetic olivine finds in metallurgy and the industry of heat-resistant materials. However, cases of the use of synthetic olivines and olivines from technogenic slags are known. From synthetic olivine, entering the jewelry market, forsterite is known - colorless and blue-violet (often used for tanzanite imitation).
As imitations of peridot are used as follows:
- synthetic minerals with a similar color (fianit, garnets, spinel, corundum);
- doublets and triplets: glass + natural or synthetic olivine;
- color glass;
