Peridot

Peridot, sometimes called chrysolite, is a yellow-green transparent variety of olivine. Peridot is one of the few gemstones that occur in only one color.

Peridot can be found in mafic and ultramafic rocks occurring in lava and peridotite xenoliths of the mantle. The gem occurs in silica-deficient rocks such as volcanic basalt and pallasitic meteorites. Along with diamonds, peridot is one of only two gems observed to be formed not in Earth's crust, but in the molten rock of the upper mantle. Gem-quality peridot is rare on Earth's surface due to its susceptibility to alteration during its movement from deep within the mantle and weathering at the surface. Peridot has a chemical formula of (Mg,Fe)2SiO4.

Etymology

The origin of the name peridot is uncertain. The Oxford English Dictionary suggests an alteration of Anglo–Norman pedoretés (classical Latin pæderot-), a kind of opal, rather than the Arabic word faridat, meaning "gemstone".

The Middle English Dictionary's entry on peridot includes several variations: peridod, peritot, pelidod and pilidod — other variants substitute y for letter i used here.

The earliest use of the word in English is possibly in the 1705 register of the St. Albans Abbey: The dual entry is in Latin with the translation to English listed as peridot. It records that on his death in 1245, Bishop John bequeathed various items, including peridot gems, to the Abbey.

Cultural history

Peridot has been prized since the earliest civilizations for its claimed protective powers to drive away fears and nightmares, according to superstitions. There is a superstition that it carries the gift of "inner radiance", sharpening the mind and opening it to new levels of awareness and growth, helping one to recognize and realize one's destiny and spiritual purpose.

In ancient times, peridot was believed to be a gift from Mother Nature, to celebrate the annual creation of a new world. National leaders who publicly wore peridot were considered kind, just, and wise. Peridot has been prized since the earliest civilizations for its supposed protective powers, which superstitiously ward off fears and nightmares. Some superstitious people believe it carries the gift of "inner splendor," sharpening the mind and opening it to new levels of awareness and growth, helping us recognize and realize our destiny and spiritual purpose. There is no scientific evidence for such claims.

In ancient times, as well as in the Middle Ages, people believed that the cosmos was reflected in precious stones. Peridot is assigned the name of the planet Saturn. The esoteric movement revived this ancient belief, and the jewelry industry made it another marketing tool to promote certain gems.

The ancient Egyptians mined peridot on the Red Sea island of Zabargad, the source of many of the fine peridots in the world's museums. The Egyptians called it the "jewel of the sun." Today, this gem is still highly prized for its yellowish-green hues.

Peridot is mentioned in the Bible under the Hebrew name "pitdah." Peridot gemstones, along with other gemstones, were used in the legendary breastplates of the Jewish high priests, artifacts that are still sought after today. Legend has it that peridot was Cleopatra 's favorite gemstone. The Crusaders brought peridot to Central Europe, where it is found in many medieval churches, such as Cologne Cathedral. During the Baroque period, the deep green gemstone enjoyed another brief period of prosperity before once again being forgotten.

Peridot olivine is the birthstone for the month of August. Peridot has often been mistaken for emerald beryl and other green gems. Noted gemologist G.F. Kunz discussed the confusion between beryl and peridot in many church treasures, most notably the "Three Magi treasure" in the Dom of Cologne, Germany.

Napoleon used peridot to assure Empress Josephine of his eternal love and admiration, which happened before he annulled their marriage.

Throughout history, peridot has been confused with other gemstones, including emerald. Many "emeralds" in royal treasures have turned out to be peridots.

The largest cut peridot was found on Zabargad Island, weighs 319 carats, and belongs to the Smithsonian Institution in Washington, D.C.

Appearance

Peridot is one of the few gemstones that occur in only one color: an olive-green. The intensity and tint of the green, however, depends on the percentage of iron in the crystal structure, so the color of individual peridot gems can vary from yellow, to olive, to brownish-green. In rare cases, peridot may have a medium-dark toned, pure green with no secondary yellow hue or brown mask. Lighter-colored gems are due to lower iron concentrations.

Mineral properties

Historically, the volcanic island of Zabargad (St. John) in the Red Sea, east of Egypt, is the most important deposit which was exploited for 3,500 years, abandoned for many centuries, rediscovered in 1900 and has been fully exploited since. The most important deposits today are in Pakistan (Kashmir region and Pakistan- Afghanistan border region), where peridots are considered to be of the finest quality. Beautiful material is also found in upper Myanmar and Vietnam. Other deposits have been found in Australia (Queensland), Brazil (Minas Gerais), China, Kenya, Mexico, Norway (north of Bergen), South Africa, Sri Lanka, Tanzania and the United States (Arizona and Hawaii). 

Crystal structure

The molecular structure of peridot consists of isomorphic olivine, silicate, magnesium and iron in an orthorhombic crystal system. In an alternative view, the atomic structure can be described as a hexagonal, close-packed array of oxygen ions with half of the octahedral sites occupied by magnesium or iron ions and one-eighth of the tetrahedral sites occupied by silicon ions.

Surface property

Oxidation of peridot does not occur at natural surface temperature and pressure but begins to occur slowly at 600 °C (870 K) with rates increasing with temperature. The oxidation of the olivine occurs by an initial breakdown of the fayalite component, and subsequent reaction with the forsterite component, to give magnetite and orthopyroxene.

Occurrence

While olivine is a common mineral, gem-quality peridot is rare—a consequence of the mineral's chemical instability at Earth's surface. Small peridot crystals are also found in volcanic basalt and meteorites. 

The earliest known mining of peridot occurred around the year 70 on St. John's Island in the Red Sea. Most early peridot stones came from there, and peridot is still mined there on a small scale today. Later, large peridot stones (20 to 40 carats) of excellent quality were mined, particularly in Myanmar (Burma), but since that country's independence, exports have largely ceased. 

Today, most peridot is mined in the United States, with a total value of $1.3 million in 1993. 80% to 95% of the world's peridot comes from the Peridot Mesa in the San Carlos Apache Indian Reservation in Arizona. Other US deposits are found in New Mexico (Buell Park, Kilbourne Hole, and Potrillo Mar). Other peridot mining areas include other locations in the United States, China, Sri Lanka, and Pakistan, where new deposits were discovered in 1994. 

Geologically

Olivine, of which peridot is a type, is a common mineral in mafic and ultramafic rocks, often found in lava and in peridotite xenoliths of the mantle, which lava carries to the surface; however, gem-quality peridot occurs in only a fraction of these settings. Peridots can also be found in meteorites.

Peridots can be differentiated by size and composition. A peridot formed as a result of volcanic activity tends to contain higher concentrations of lithium, nickel and zinc than those found in meteorites.

Olivine is an abundant mineral, but gem-quality peridot is rather rare due to its chemical instability on Earth's surface. Olivine is usually found as small grains and tends to exist in a heavily weathered state, unsuitable for decorative use. Large crystals of forsterite, the variety most often used to cut peridot gems, are rare; as a result, peridot is considered to be precious.

In the ancient world, the mining of peridot was called topazios then, on St. John's Island, in the Red Sea began about 300 BCE.

The principal source of peridot olivine today is the San Carlos Apache Indian Reservation in Arizona. It is also mined at another location in Arizona, and in Arkansas, Hawaii, Nevada, and New Mexico at Kilbourne Hole, in the US; and in Australia, Brazil, China, Egypt, Kenya, Mexico, Myanmar (Burma), Norway, Pakistan, Saudi Arabia, South Africa, Sri Lanka, and Tanzania.

In meteorites

Peridot crystals have been collected from some pallasite meteorites. The most commonly studied pallasitic peridot belongs to the Indonesian Jeppara meteorite, but others exist such as the Brenham, Esquel, Fukang, and Imilac meteorites. Pallasitic (extraterrestrial) peridot differs chemically from its earthbound counterpart, in that pallasitic peridot lacks nickel.

Use as a gemstone

Peridot is light green in color, ranging from yellow-green to olive-green. Brownish varieties are also known. Peridot is sometimes confused with emerald and other green gemstones, including chrysoberyl, diopside, moldavite, and varieties of garnet (demantoid), quartz (prasiolite, praseme), and tourmaline (cromolite, verdelite). 

Peridot (as pitdah) is already mentioned in the Old Testament, and in ancient Rome the stone was prized as the "evening emerald" because it is easily illuminated by lamplight. In the Middle Ages, peridot was often used in church decorations; peridot stones were probably brought to Europe by Crusaders. The stone was believed to have healing powers, as well as the power to drive away evil spirits. 

Gemology

Orthorhombic minerals, like peridot, have biaxial birefringence defined by three principal axes: α, β, and γ. Refractive index readings of faceted gems can range around α = 1.651, β = 1.668, and γ = 1.689, with a biaxial positive birefringence of 0.037–0.038. With decreasing magnesium and increasing iron concentration, the specific gravity, color darkness and refractive indices increase, and the β index shifts toward the γ index. Increasing iron concentration ultimately forms the iron-rich end-member of the olivine solid solution series fayalite.

A study of Chinese peridot gem samples determined the hydro-static specific gravity to be 3.36. The visible-light spectroscopy of the same Chinese peridot samples showed light bands between 493.0–481.0 nm, the strongest absorption at 492.0 nm.

The largest cut peridot olivine is a 310-carat (62-gram) specimen in the gem collection of the Smithsonian Museum in Washington, D.C.

Inclusions are common in peridot crystals but their presence depends on the location where it was found and the geological conditions that led to its crystallization.

Primary negative crystals – rounded gas bubbles – form in situ with peridot, and are common in Hawaiian peridots.

Secondary negative crystals form in peridot fractures.

"Lily pad" cleavages are often seen in San Carlos peridots, and are a type of secondary negative crystal. They can easily be seen under reflected light as circular discs surrounding a negative crystal.

Silky and rod-like inclusions are common in Pakistani peridots.

The most common mineral inclusion in peridot is the chromium-rich mineral chromite.

Magnesium-rich minerals also can exist in the form of pyrope and magnesiochromite. These two types of mineral inclusions are typically surrounded "lily-pad" cleavages.

Biotite flakes appear flat, brown, translucent, and tabular.

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