PERIDOT
- VARIETY OF: Olivine , (Mg, Fe)2SiO4.
- USES: gemstone.
- BIRTHSTONE FOR: August
- COLOR: shades of green usually from
yellow-green to greenish yellow.
- INDEX OF REFRACTION: 1.630 - 1.695
- BIREFRINGENCE: 0.037
- HARDNESS: 6.5 - 7
- CLEAVAGE: usually not discernible
- CRYSTAL SYSTEM: orthorhombic
Peridot is the gem variety of olivine. Olivine, which
is actually not an official mineral, is composed of two
minerals: fayalite and forsterite. Fayalite is the iron
rich member with a pure formula of Fe2SiO4.
Forsterite is the magnesium rich member with a pure
formula of Mg2SiO4.
Olivines's formula is written as (Mg, Fe)2SiO4
to show the substitution of the magnesium and iron.
Peridot is usually closer to forsterite than fayalite in
composition although iron is the coloring agent for
peridot. The best colored peridot has an iron percentage
of less than 15% and includes nickel and chromium as trace
elements that may also contribute to the best peridot
color.
Peridot is perhaps derived from the French word peritot
which means unclear, probably due to the inclusions and
cloudy nature of large stones. It could also be named from
the Arabic word faridat which means gem. In either
case, peridot has been mined as a gemstone for an
estimated four thousand years or better, and is mentioned
in the Bible under the Hebrew name of pitdah.
Peridot gems along with other gems were probably used in
the fabled Breastplates of the Jewish High Priest,
artifacts that have never been found. The Greeks and
Romans referred to peridot as topazion and topazius
respectively and this name was later given to topaz, to
end the confusion with the two gems. Historical legend has
it that peridot was the favorite gemstone of Cleopatra.
Pliny wrote about the green stone form Zagbargad Island in
1500 B.C.. Even until recently have jewelers used the term
"chrysolite" (Latin for golden stone) in
referring to peridot gems for some reason. This term has
also been used to refer to other gemstones, of a more
golden color.
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