Peridot – August Birthstone by Wayne Farley, July 2017

Peridot(shown below) is the name for gem-quality olivine. Olivine (also known in the past as chrysolite) is a solid-solution series silicate mineral with the general formula of (Mg, Fe)2SiO4. The magnesium end member is forsterite (Mg2SiO4), and the iron end member, fayalite (Fe2SiO4). Peridot falls on the forsterite side, and gets it's lime-green, yellow-green, or olive-green color from the small amount of iron within the crystal. The green color is sometimes enhanced by small amounts of nickel and chromium in the crystal lattice. Hausel (Ref-6) mentions rare red and orange-red varieties of peridot. 

Peridot Color:The best peridot color is olive-green, and has an iron content of 12-15%.  As iron increases above the ideal content, the crystals change to a greenish-brown.  The greener the color, the more valued is the stone. A rare variety of bright green peridot from Hawaii contains some chromium. 

Pure forsterite is colorless and pure fayalite black. Peridot falls within the orthorhombic crystal system, has a hardness of 6.5 to 7, a specific gravity of 3.27 to 3.37, and a refraction index of 1.654-1.690. It is double refractive.  

  Olivine is one of the most abundant natural minerals in the earth's crust, occurring as small granules in basic and ultra-basic rocks, and as xenoliths in volcanic eruptions of upper mantle derived material.  The upper mantle starts at the Moho, or base of the crust around, 7 to 35 km (4.3 to 21.7 mi) downward to 410 km (250 mi).     

Gem miners find peridot rough as irregular nodules (rounded rocks with peridot crystals inside) in some lava flows in the United States, China, and Vietnam and, very rarely, as large crystals lining veins or pockets in certain types of solidified molten rock. Sources for the latter include Finland, Pakistan, Myanmar, and the island of Zabargad.  

Geologists believe both types of peridot deposits relate to the spreading of the sea floor that occurs when the earth’s crust splits, and rocks from its mantle are pushed up to the surface. Sometimes—as in Myanmar— these rocks can be altered, deformed, and incorporated into mountain ranges by later earth movements. 

Rarely, peridot can have an extraterrestrial source, being contained in pallasite meteorites that have fallen to earth. (Ref.1 & 10)  A pallasite peridot is considered large if above 1/3 caret. Stinson's, offer's pallasite peridots for sale, the largest being 1.3 carets.  Gems cut from pallasites tend to be pale, grayish, and foggy.   

Peridot History:Peridot is a gemstone with a fascinating history, both in nature and culture. It is one of the oldest known gemstones, with ancient records documenting the mining of peridot from as early as 1500 B.C. It is a gem especially connected with ancient Egypt, and some historians believe    that the famous emeralds of Cleopatra were actually peridot gems.

During the middle ages, the Crusaders brought peridots back to Europe; some of these gems are preserved in European Cathedrals.

  Peridot was highly prized during the latter part of the Ottoman Empire (1300-1918). Turkish sultans amassed the world's largest collection of the gems. In Istanbul's Topkapi Palace Museum, there is a gold throne decorated with 955 peridot cabochons and on jeweled boxes, and literally thousands of loose peridots.  

 The main source of peridot in the ancient world was Topazos Island, now known as Zabargad or St. John's Island, in the Egyptian Red Sea. The island was discussed in Pliny the Elder's Natural History (23-79 A.D.) as having been explored in the fourth century BC. Peridot was probably originally referred to as topaz; only much later did the name come to be applied to the gemstone we know today as topaz. Peridot has been mined on Zabargad for over 3,500 years. Incidentally, the exact location of the island was lost for several centuries and was only rediscovered in 1905. The tiny island, often shrouded in fog, is located about 35 miles off the Egyptian coastal port of Berenica. Mining ended on Zabargad around the time of World War II. Today there are five major sources of peridot. Very fine specimens come from Burma, and new material from Pakistan has generated considerable excitement in the gemstone world. Arizona and New Mexico in the USA are important producers of commercial grade peridot. Additionally, Vietnam and China have become increasingly important suppliers.  

Peridot Mythology: Peridot was believed to be able to dissolve enchantments. To exert its full potential, the stone was set in gold. If it was intended to provide protection from evil spirits, it had to be pierced, strung on the hair of a donkey, and worn on the left arm. As a medical remedy, it was powdered to cure asthma. Additionally, holding a peridot under the tongue was supposed to lessen the thirst of a person suffering from fever. The high priest's breastplate, which is described in the Book of Exodus, includes a stone for each of the twelve tribes of Israel, one of which is believed to have been peridot.  

 
 Olivine (peridot) Geology: Olivine is the name of a group of rock-forming minerals that are typically found in mafic and ultramafic igneous rocks such as basalt, gabbro, dunite, diabase, and peridotite.  
 Peridotites are ultramafic rocks with no plagioclase feldspar and which olivine is the dominate mineral. In some cases olivine may be the only mineral and the rocks are known as dunites or olivinites.  

 Crystals of olivine are sometimes formed during the metamorphism of a dolomitic limestone or dolomite. The dolomite contributes magnesium, and silica is obtained from quartz and other impurities in the limestone. When olivine is metamorphosed, it is transformed into serpentine. 

 Hausel , in his geological mapping of Wyoming, indicates an association of olivine (peridot) with kimberlite-peridotite and lamproite. Lamproiteis a porphyritic ultrapotassic igneous rock dominated by phlogopite, richterite, olivine, diopside, leucite, and sanidine; and is one of the rarest rocks on earth. These rocks are found in the Leucite Hills of southwestern Wyoming, north of the towns of Rock Springs and Superior, where Lamproite forms rare high potassium (ultrapotassic) volcanoes, flows, and plugs. Hausel states in his book (Ref-6), that he found more than 13,000 carets of olivine (mostly peridot) in two anthills along the western flank of Black Rock during his investigation of the area for diamonds. The largest was 12 mm in length. Well preserved dunite and peridotite xenoliths at Black Rock contain larger olivine grains (as large as 0.25 inch). Hausel states that much of the peridot from this area is transparent and flawless. 

Olivine (peridot) locations: Major world locations of forsterite mineralization are listed in “Moore's Compendium of Mineral Discoveries”. 

A noticeable exception is the lack of mention of the San Carlos Indian Reservationperidot deposit in Arizona. For over a century, the San Carlos Apache Reservation in Gila County, Arizona, has been a major source of gem-quality olivine (peridot) in sizes that are very suitable for jewelry. Yet very little has been published about the principal gem locality on the reservation, Peridot Mesa, or the peridot that is mined there by the Apache Indians, who have exclusive rights to the deposit. The peak period of mining was from 1904 to 1909. 

Exceptional Peridot Crystals and Gemstones: The Sinkankas Symposium book (Ref-10) displays a number of exceptional specimens from the William F. Larson collection. Included are crystals up to 4.8 cm long, and gemstones up to 232.93 carets. The Larson collection, and other exceptional peridots, can be observed on Google images, or the John Betts virtual museum. (Ref.-13) 

 Other Green Minerals: Other green minerals that can be confused for peridot are: beryl, chrysoberyl, demantoid, diopside, moldavite, prasiolite, prehnite, sinhalite, emerald, synthetic spinel, tourmaline, and idicrase. The strong double refraction in peridot, however, is an important distinguishing mark for identification. (Ref.-14)
 The “Color Encyclopedia of Gemstones”, 1987 (Ref.-15), list the following locations for peridot.   

1. Mogok, Burma  

2. Zabargab, Egypt 

3. Norway  

4. Tanzania  

5. Arizona (light green)  

6. Arizona  

7. Mexico (light green)  

8. Mexico (brown)  

9. New Mexico  

10. Sri Lanka (olive)  

11. Sti Lanka (almost colorless)  

12. Kenya (yellowish)  

13. Kenya (brownish)  

A more recent report on the source of peridot by Wise 2016 (Ref.-17) states the following: “Pakistan is the major supplier of lager fine peridot. Most other sources do not currently produce much of note. Currently there is some peridot coming out of Burma and production from San Carlos Apache Reservation in East Central Arizona is mostly in gems less than two carats.” Hawaii’s Green Sands:  Mahana Beach on Hawaii’s Papakolea coast is one of only two green sand beaches in the world. The beach sand on the Big Island’s undeveloped southern tip is rich in the mineral olivine (gem-quality olivine is known as peridot, the August birthstone). Olivine is a common mineral component of Hawaiian lavas and one of the first crystals to form as magma cools.  

Locals refer to peridot as the “Hawaiian Diamond,” and small peridot stones are sold as “Pele’s tears” in honor of Pele, the goddess of volcanoes. In ancient Hawaiian chants, Pele was described as “She-who-shapes-the-sacred-land,” and her temper was known to be both as abundant and dangerous as the lava. 

Those daring enough to take the three-mile hike through lava fields to the remote beach at the crescent-shaped bay of Pu’u Mahana, will be treated to a display of one of nature’s crowning achievements — a green beach that appears surreal against the backdrop of steely grey cliffs, turquoise blue ocean and bright blue sky. The abundance of olivine crystals filling the beach comes from the eroded cutaway interior of Pu’u Mahana, a volcanic cone produced more than 49,000 years ago by the explosive combination of lava and groundwater. 

References:

http://www.gia.edu/peridot-history-lor  

http://www.geology.com  

http://www.gemselect.com  

 “Gemstones and Their Origins”, Peter C.   Keller, 1990  

“Identification of Gemstones” Michael O'Donoghue and Louis Joyner, 2003  

 “Gems, Minerals & Rocks of Wyoming, W.   Dan Hausel, 2009  

“Rocks & Minerals”, Pat Bell & David Wright, 1985 

“Moore's Compendium of Mineral Discoveries”, Thomas P. Moore, 2016.

https://en.wikipedia.org/wiki/Lamproite  

“Twelfth Annual Sinkankas Symposium, Peridot, & Uncommon Green Gem Minerals”,   Roger Merk, et al, 2014.   
  http://www.gemservice.com/Meteorite.htm    

”Gems & Crystals”, American Museum of Natural History, George Harlow, 2015