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Peridot

peridot
A group of mixed faceted peridot

Gem quality specimens of the mineral olivine are called peridot by gemmologists. Peridot has an olive or bottle green colour due to the presence of iron, and a distinctive oily or greasy lustre. It has a high birefringence, so doubling of the back facets can easily be seen in larger specimens when viewed from the front. Good-quality crystals are very rare.

Peridot is found on St. John’s Island (Egypt), in China, Burma, Brazil, Hawaii and Arizona (USA), Australia, South Africa and Norway.

The Crusaders brought peridot to Europe in the Middle Ages, transporting stones from St. John’s Island in the Read Sea, where it had been mined for over 3,500 years.

Peridot Pictures

Tumbled peridot

Cause of Color : Iron – due to ferrous iron (ideally 12% – 15%). Presence of chromium gives a brighter green.
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.


Crystal System / Forms :

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


Cuts & Uses :

Facetted cut, cabochon, beads, carvings, etc.


Dispersion : 0.020
Hardness : 6.5 - 7 (Fayalite - Forsterite)
Lustre : Oily to vitreous.
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.


Optic Character :

Anisotropic, D.R.; Biaxial positive.


Pleochroism : Weak
Refractive Index / Birefringence :

1.654 – 1.690 / 0.036. Range: 1.640 – 1.700


Simulants (with 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)


Sources :

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


Specific Tests :
  • Doubling is seen.
  • Brittle stone.
  • Attacked by acids.

Spectrum :

Bands at 493nm, 473nm & 453nm.


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