Peridot

Gem quality specimens of the mineral olivine are known as peridot by gemmologists. Peridot comes with an olive or bottle green colour because of the presence of iron, along with a distinctive oily or greasy lustre. It features a high birefringence, so doubling of the rear facets may be easily observed in larger specimens when viewed in the front. Good-quality crystals are extremely rare.

Properties

Chemical Composition: 

Silicate of magnesium and iron where iron is generally 12% - 15%, Cr, Mn & Ni may also be present. (MgFe)₂SiO₄

Classification / Type: 

Mineralogically known as Olivine, peridot belongs to the isomorphous Forsterite (Mg) - Fayalite (Fe) series.

Colors / Varieties: 

• Forsterite: green, pale lemon yellow.
• Fayalite: green, brown, yellowish green, olive green.

Crystal System / Forms: 

Orthorhombic System / Striated prisms, rolled pebbles, crystals are rare.

Hardness: 

6.5 - 7 (Fayalite - Forsterite)

Specific Gravity: 

3.34

Cleavage / Fracture: 

Imperfect to weak / Conchoidal fracture.

Optic Character: 

Anisotropic, D.R.; Biaxial positive.

Lustre: 

Oily to vitreous.

Refractive Index / Birefringence: 

1.654 - 1.690 / 0.036. Range: 1.640 - 1.700

Pleochroism: 

Weak

Dispersion: 

0.020

Magnification: 

Lily pad inclusions are halos with a crystal (chromite or chrome spinel) at the centre, glass balls that look like bubbles (Hawaiian), rectangular biotite crystals, doubling.

U.V. Fluorescence: 

Inert

Spectrum: 

Bands at 493nm, 473nm & 453nm.

Cause of Color: 

Iron - due to ferrous iron (ideally 12% - 15%). Presence of chromium gives a brighter green.

Treatment (Enhancement): -

Specific Tests & Remarks: 

• Doubling is seen.
• Brittle stone.
• Attacked by acids.

Synthesis: -

Simulants (with key separation tests): 

Sinhalite (pleochroism, spectrum), Diopside (R.I., spectrum), Tourmaline (optic figure, R.I., S.G., pleochroism), Apatite (R.I., birefringence, inclusions, spectrum), Fluorite (optic character), Glass (optic character), Synthetic Spinel Triplet (optic character, inclusions)

Geological Occurrence: 

Found in peridotites, in concentrations in basalt and ultrabasic rocks.

Sources: 

Hawaii, Zeberged (Egypt), Isle of St. John, Myanmar, Pakistan, Norway, Kenya, U.S.A. (California, New Mexico and Arizona).

Cuts & Uses: 

Facetted cut, cabochon, beads, carvings, etc.

Peridot is the green variety of the Olivine Group of minerals. Olivine is not an individual mineral but a group of minerals that includes Fayalite, Forsterite, Liebenbergite, Tephroite and Laihunite. Fayalite (Mg2SiO4) is the iron rich member of the Fayalite - Forsterite mineral series, Forsterite (Fe2SiO4) is the magnesium rich member. Peridot (Mg2Fe2SiO4) is the green variety of Forsterite containing both Iron (Fe) and magnesium (Mg). Peridot's color can vary from pale to rich green (for which Peridot is famous) to brown. The considerable variation in shades of green to brown is dependant on locality of origin and amount of ferrous iron. The ideal green Peridot has an iron content of about 12-15%. More iron results in a muddy color. Very bright green Peridot may also contain traces of chromium.

Peridot is not a recognized mineral name. It's hard to believe that Peridot is the equivalent of a trade name, not a recognized mineral name since it is one of the most popular and well known gems in the world. Peridot has been a popular gem throughout history.

Peridot is available from many localities worldwide including these well known sources: Mogok, Sagaing District, Mandalay Division, Burma (Myanmar); St John's Island (Zagbargad; Zebirget; Topazios), Red Sea, Egypt; Naran-Kagan Valley, Kohistan District, North-West Frontier Province, Pakistan; and Peridot Mesa, San Carlos, San Carlos Indian Reservation, Gila Co., Arizona, USA