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August Birthstone - Peridot
Physical and Chemical Properties
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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