Peridot

Etymology

• The origin of the name 'peridot' is uncertain.
• The Oxford English Dictionary suggests it may be an alteration of Anglo-Norman 'pedoretés', a kind of opal.
• The Middle English Dictionary lists several variations of 'peridot'.
• The earliest use of 'peridot' in English dates back to 1705.
• Bishop John bequeathed peridot to the St Albans Abbey in 1245.

Appearance

• Peridot occurs in only one color: olive-green.
• The intensity and tint of the green depend on the percentage of iron in the crystal structure.
• Peridot gems can vary in color from yellow to olive to brownish-green.
• In rare cases, peridot may have a pure green color with no secondary hues.
• Lighter-colored peridot gems have lower iron concentrations.

Mineral properties

• Peridot has an orthorhombic crystal structure.
• The molecular structure consists of isomorphic olivine, silicate, magnesium, and iron.
• Peridot does not oxidize at natural surface temperature and pressure.
• Oxidation begins to occur slowly at 600°C.
• The oxidation of olivine leads to the formation of magnetite and orthopyroxene.

Occurrence

• Peridot is found in mafic and ultramafic rocks, lava, and peridotite xenoliths of the mantle.
• Gem-quality peridot is rare due to its susceptibility to weathering on Earth's surface.
• Peridot can be found in volcanic basalt, pallasitic meteorites, and silica-deficient rocks.
• The main source of peridot olivine is the San Carlos Apache Indian Reservation in Arizona.
• Peridot is also mined in other locations such as Arkansas, Hawaii, Nevada, and various countries worldwide.

Gemology

• Peridot has high birefringence, leading to doubling of facet junctions under magnification.
• Refractive index readings for peridot include α = 1.653, β = 1.670, and γ = 1.689.
• The specific gravity of peridot is approximately 3.36.
• Peridot's color variation is influenced by the iron concentration.
• Inclusions commonly found in peridot include chromite, pyrope, magnesiochromite, and biotite flakes.

Peridot Data Sources