August Birthstone - Peridot

Physical and Chemical Properties

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Chemical composition

Peridot is a gem variety of the mineral Chrysolite or Olivene and its chemical formula is given by: (Mg,Fe)2SiO4. The ratio of Magnesium and Iron in the crystal is highly variable and the name Forsterite (Fo) is applied to Magnesium-rich/ Iron-poor crystals whereas the name Fayalite (Fa) is applied to Magnesium-poor/ Iron-rich crystals. Crystals are often flattened and much peridot is found in granular masses or embedded grains in a finer grained basic igneous rock such as basalt or gabbro. Peridot has a distinct cleavage (breakage along preferred planes) and a conchoidal (shell-like) fracture. It ranges from about 6.5 to 7 on Mohs hardness scale. Peridot is fairly dense with a specific gravity ranging from about 3.27 to 3.37. The refractive indexes (the numerical measures of how much a light beam is bent and slowed down when it enters a substance) of peridot range from about 1.654 to 1.690. Peridot has three refractive indexes, two of which remain fixed and one that is variable and numerically between the upper and lower index. The birefringence of peridot is fairly high: 1.690 - 1.654 = 0.036. This fact is very valuable to the jeweler or gemologist for when one views a faceted peridot through the table of the stone, the junctions of adjacent facets are strongly doubled. Inclusions in peridot are also strongly doubled. Peridot may have small inclusions of biotite (brown), chromite (black), pyrope garnet (dark red), spinel (tiny octahedra) or liquid and gas-filled inclusions that resemble fried eggs. The strong doubling of facet junctions and inclusions as well as the pale yellowish green color are very characteristic of peridot.

Colors

Peridot has a distinctive olive or bottle green color that sometimes has a brownish tinge. The green color is due to the iron presence, while the brownish tint is the result of higher iron concentration.

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