Cochise College            Student Papers in Geology     
Geology Home Page                    physical geology  historical geology  planetary  gems

Roger Weller, geology instructor                             

wellerr@cochise.edu
 


Birthstones
Michael Schweska
Physical Geology
Spring 2005
 

                                    August Birthstone-Peridot

 
Peridot is the official birthstone for August as adopted by the American National Association of Jewelers in 1912. It is also the stone for the Zodiac sign of Libra. Peridot is suggested as a gem to give on the 16th wedding anniversary. Peridot is 6.5 - 7 on the Mohs scale of hardness. Gem quality Peridot comes from Zagbargad Island in the Red Sea, Myanmar "formerly known as Burma), Pakistan, Brazil, Germany, Mexico, Australia, Arizona and Hawaii, USA. The best quality peridot has historically come either from Myanmar or Egypt but in 1994 a new deposit of peridot was discovered in Pakistan which produces some of the finest stones. Large crystals have been found in this area, one stone was more than 300 carats. The gem material found in Arizona, which is approximately 85% of the world's production, is of lesser quality, because it is an abundant source, it is much more affordable. Large peridots, close to 200 carats in size, decorate the shrine of the three Magi at the Cologne Cathedral. Peridot Click here for a picture in natural form. Click here for more faceted and natural specimens.

Description:

Peridot ranges in color from light yellow-green to the intense bright green of new spring grass to olive. Because of the way Peridot splits and bends the rays of light passing through it, it has a velvety appearance, a rich glow, and a slightly greasy luster. The purer green a peridot is the higher the value. Any tinges of brown or visible flaws greatly diminish the price. Throughout time, Peridot has been confused with many other gemstones, even Emerald. Many Emeralds of royal treasures have turned out to be Peridots. And although Peridot is distinctly a different shade of green, many jewelers refer to Peridot as "Evening Emerald". Emerald is a dark green as opposed to a yellow green and always contains inclusions. Other green gemstones confused with Peridot include Apatite (which is much softer), Green Garnets (have no double refraction), Green Tourmaline and Green Sinhalite (both of which are strongly pleochroic), Moldavites (no double refraction, an unusual olive green gem that comes from meteorites called pallasites, Moldavite is found in the Czech Republic and believed to have arrived from space in a meteor about 14.8 million years ago) and Green Zircon (significantly heavier). All of these gemstones rarely have as nice a yellow component to their green color as does most peridot, but darker green peridot can be confusing when good crystal form is not discernible. It is not clear whether the word Peridot comes from the Arabic word "faridat", which means "gem" or if it is derived from the French word "peritot" which means "unclear". In the 18th century, the French were the first to call this yellowish-green stone Peridot. Before this time, Peridot was called Topaz. Peridot has been mined as a gemstone for an estimated four thousand years and is mentioned in the Bible under the Hebrew name of "pitdah".

Chemistry:

Peridot pronounced (pair-a-doe) is the gem variety of the mineral Chrysolite or Olivine, the formula is written as (Mg, Fe)2SiO4 to show the substitution of the magnesium and iron. 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. 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.

The Olivine Group is a term that is sometimes incorrectly applied to just two minerals that are often lumped together and simply called olivine. The two minerals are fayalite and forsterite and are perhaps best referred to as the Olivine Series. Although olivine is not an official mineral name in itself, it is a term that is used to denote intermediate specimens between fayalite and forsterite. The true Olivine Group is more inclusive and is a group of similarly structured orthorhombic nesosilicates. The structure of the Olivine Group is composed of a layered closest-packed oxygen framework with the silicon ions occupying tetrahedral sites and the metal ions occupying octahedral sites called M1 and M2 The M1 site is a slightly distorted octahedron. The silicon tetrahedral sites or silicate tetrahedrons, SiO4, are not adjacent to each other and therefore independent to each other making them true nesosilicates.

These are the members of the Olivine Group:
  • Fayalite (Iron Silicate)
  • Forsterite (Magnesium Silicate)
  • Glaucochroite (Calcium Manganese Silicate)
  • Kirschsteinite (Calcium Iron Silicate)
  • Laihunite (Iron Silicate)
  • Liebenbergite (Nickel Magnesium Silicate)
  • Monticellite (Calcium Magnesium Silicate)
  • Olivine (Magnesium Iron Silicate)
  • Tephroite (Manganese Silicate)

Legends, Myths and Healing Properties:

PeridotPeridot gems were probably used in the fabled Breastplates of the Jewish High Priest and historical legend says that Peridot was the favorite gemstone of Cleopatra, although at the time they were called Emeralds. It is thought to bring the wearer good luck, peace, and success. Its powers include health, protection, and sleep. The advantages of Peridot are to attract love and calm anger while also soothing nerves and dispelling negative emotions. Peridot is considered a tonic for the whole body and protects the wearer from negativity. It is associated with stress reduction and relaxation. Egyptians used Peridot to clean and heal the heart. Powdered Peridot has been used to cure asthma and a Peridot placed under the tongue of someone in the grip of a fever is said to lessen his or her thirst. Legend has it that drinking from a Peridot goblet can increase the potency of medicines.


 

 
 


This page is for educational purposes only. Reproduction of this material for any purpose other than educational is not granted.
All rights are reserved by original sources.
Smithsonian Institute 1998-2005 Bernardine Fine Art Jewelry. 1999-2004 Jewelry Central, JC Store.
2004 Amethyst Galleries, Inc. 1995-2005 Loretta Elaine's Gems for Friends.