SCIENCE OF STONES: PERIDOT

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PHOTOGRAPHED BY SAM GOLD

 

MINERAL: Olivine // CHEMICAL COMPOSITION: Mg2SiO4-Fe2SiO4 // COLOR: Green/Yellow // REFRACTIVE INDEX: 1.65 - 1.69 // BIREFRINGENCE: +0.036 // SPECIFIC GRAVITY: 3.2 - 3.4 // MOHS HARDNESS: 6.5 - 7

 

During ancient times, people thought peridot was a gift from Mother Nature, a celebration of the new world. It was said to contain magical and healing properties to ward off nightmares.

Although peridot can be pronounced both with and without the "t" at the end, most professionals in the gem trade pronounce the "t". Peridot has conflicting name origins. Some state it is named after the French word “peritot”, meaning gold, after the early discoverers found many examples of the mineral veered towards this color. Other scholars agree that the word is derived from the Arabic faridat which means “gem,” yet others believe it’s rooted in the Greek word peridona, meaning “giving plenty.” Perhaps that’s why peridot is associated with prosperity and good fortune.

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Peridot was born early in the solidification of the Earth as magma slowly cooled to form Mafic and Ultramafic igneous rock billions of years ago. Parts of the magma which cooled particularly slowly created large and clear specimens of peridot with the richest deposits settling in Egypt, Burma, Hawaii and Arizona where it’s been brought to the earth’s surface through volcanic eruptions.

A magnesium-rich, gem-quality variety of olivine, the formula for peridot approaches Mg2SiO4 (forsterite). Olivine is a silicate mineral with the formula of Mg2SiO4 -  Fe2SiO4 (fayalite) . Despite olivine’s abundance, gem quality peridot is rare in comparison. This is due to the mineral's chemical instability on the Earth's surface. Olivine is usually found as small grains, and tends to exist in a heavily weathered state, unsuitable for jewelry. Papakolea Beach in Hawaii with its green sand can attest to this.

Mafic and ultramafic rock are where olivine is most commonly found. Mafic rocks are silica-poor igneous (formed when lava cools) rocks rich in magnesium and iron. Ultramafic rocks are also igneous but contain higher levels of iron oxide, lower silica contents, less potassium and 90% or more of their makeup is based on the same mineral base that creates mafic rock. Because of it’s igneous roots, peridot is often found in lavas and the ultramafic rock peridotite, which magma carries to the surface through volcanic eruptions. By the time it reaches the surface, the peridotite may be encased within another rock that has formed around it. Geologists call these xenoliths. Gem quality peridot occurs in only a fraction of these. In 1749, a meteorite that landed in a desolate area of Siberia contained peridot crystals large enough to be set into jewelry. Even though many different gems are found in meteorites, peridot is the only one found large enough to make jewelry although few specimens have the tenacity to withstand the jewelry making process.

Peridot has a storied history beyond being August’s birthstone. Peridot is the stone given to celebrate the 16th year of marriage. In theology, the second gemstone in Aaron’s breastplate was believed to be peridot. There is an argument that the seventh foundation stone for the New Jerusalem of the Bible was peridot. Others note the power of the gem is contained in a statement made by the Bishop of Mainz about 1,100 years ago to the effect that “…in the peridot is shown true spiritual preaching accompanied by miracles.” In Hawaii, peridot symbolizes the tears of Pele, the volcano goddess of fire who controls the flow of lava.

Through medieval times, people continued to confuse emerald for peridot and vice-versa. The 200-carat gems adorning one of the shrines in Germany’s Cologne Cathedral were long believed to be emeralds, but were recently found to be peridots. In the middle ages, Europeans brought peridot stones back from the Crusades to decorate church plates and robes. Peridot was also known to ancient Hebrews and is listed both as one of the aforementioned stones used by Aaron and found in the text of the apocalypse (Revelations).

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The main source of peridot in the ancient world was Topazo Island (now Zabargad or St. John’s Island) in the Egyptian Red Sea. Peridot is the national gem of Egypt, ancient scrolls from the region record the mining of these stones from 1500 BC though they are thought to have been mined on St John’s Island since 3,500BC. Some historians believe Cleopatra’s famed emerald collection may have actually been peridot. In Ancient Egypt, peridot stones were used for carved talismans and island inhabitants were forced to collect the gems for the Pharaoh’s treasury. Ancient Egyptians knew it as “the gem of the sun.” Peridot continued to be mined on St Johns Island until as late as the 19th Century. Joel Aram, from the “Color Encyclopedia of Gemstones 2nd Edition,” writes “Zabargad is an island in the Red Sea that is often shrouded in fog, making it difficult for ancient navigators to find. The location had been lost for centuries, and was rediscovered in about 1905. The island is located 35 miles off the Egyptian coastal port of Berenica.”  In the 19th Century, the mines on Zabargad produced millions of dollars worth of peridot. After 1905, production of the gems peaked, but by the late 1930’s it tapered off to practically nothing and reached a virtual standstill in 1958, when the mines were nationalized.

Peridot mostly occurs in small, worn-down, pebble-sized specimens, weathered by tens of thousands of years of erosion in gem stone gravels. In Hawaii, the black basalt rock and beaches are studded with millions of tiny peridot grains. Although most of the peridot on Hawaii is either too small to use, or is locked in very hard rock (xenoliths) that can't be mined, some pieces of Hawaiian peridot have led to jewelry quality stones. Burma produces some good peridots that tend to be a darker shade of green. Brazil, as well as Arizona and New Mexico, also provide some fine, smaller gemstones. In addition to these locations, peridot is mined regularly in Mexico, the state of Oregon, Norway, Pakistan, Russia and Sri Lanka. The San Carlos Reservation in Arizona supplies an estimated 80 to 95 percent of the world’s peridot supply. Most of the peridot mined today is under 3 carats. Peridots of two or three carats are highly prized, and a stone from three to eight-carat stone is extremely rare. Any stones beyond eight carats are collectors or museum pieces. The American Museum of Natural History in New York City has one of the world’s foremost collections of peridot. The largest peridot discovered is a 310 carat stone housed in the Smithsonian museum.

As with all fine gems, small crystals are relatively common and larger stones seldom occur free of flaws. Due to earthquakes and volcanic eruptions, the larger peridot crystals become, the easier they flaw. Stones are subsequently cut and faceted in such a way that these imperfections may be eliminated. Typically, a 20-carat stone in the rough will only yield three or four gems of half a carat to three carats each. Today, because large stones are increasingly rare, the green for which peridot has for centuries been praised is seldom seen. This saturate green was so highly prized that one of antiquities favorite compliments to peridot was to mistake it for emerald. This emerald-looking shade of green is almost never encountered in peridots under ten carats. To find stones of such color, one had to look in Egypt and Burma. Peridot’s signature green color comes from the composition of the mineral itself—rather than from trace impurities, as with other colored stones. It is one of few gems that occur in only one color, though shades may vary from yellowish-green to olive to brownish-green, depending how much iron is present.

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Peridot is valued using the 4C’s of color, clarity, cut and carat. Peridots are transparent with a distinct oily luster. Peridot’s color can be described as yellow-green, green with a golden tone, olive or bottle green, deep chartreuse, or simply a brilliant light green. The most desirable color is a deep olive-green with no hint of yellow or brown. Deeper olive-green tones tend to be more valuable than lighter colored greens and yellowish-greens. The amount of iron present is proportional to the shade and depth of the green of a peridot stone; the deeper the green, the less iron present. Peridot is not routinely treated. While it is not particularly brilliant, the richness of its color can be exceptional and is completely natural.

Under 10x magnification, peridots will often reveal a distinctive, disk-like liquid and gas inclusion. Known as lily pad inclusions (or sometimes “lotus leaves”), these may form around spinel or biotite crystals as well as cavities. Despite most stones not receiving any sort of treatment, some cut peridots may be metal-foiled to improve their stability and color. Although older stones, have cloudy or milky inclusions, more recent gemstones on the market have few flaws and can be of exceptional quality. Most of the better-quality, calibrated material and larger single pieces on the market have no eye-visible inclusions, with tiny black spots—actually minute mineral crystals—visible under magnification.

Peridot may be cut in a wide variety of shapes such as round, oval, pear, cushion, triangle, and marquise. Cutting styles are also well represented. Brilliant cuts with triangular and kite-shaped facets, step cuts with concentric rows of parallel facets, and mixed cuts consisting of brilliant-cut crowns and step-cut pavilions are all common. Designer cuts fashioned by hand and machine are popular, as well as cabochons, beads, and carvings.

Peridot only measures 6.5 (fayalite) to 7 (forsterite) on the Mohs scale, so the raw crystal is prone to cracking during cutting. It is not a very durable stone and are sensitive to scratching from household dust (which is mostly quartz 7.5 Hardness) and can chip if bumped hard. Furthermore, they have some susceptibility to stress fractures and have been known to burst under pressure. Peridot can not stand intense heat or rapid temperature changes. Jewelers will avoid settings that place stress on peridots and will clean them only with a soft brush, mild detergent, and warm water, not steam or ultrasonics. Their sensitivity to acids, even that in perspiration means peridot jewelry should be worn against the skin only occasionally or they may lose their polish.