The August birthstone is peridot, which is a green gemstone. Peridot has been used as a birthstone for the month of August for centuries. The vivid green color of peridot is striking and has led to its popularity in jewelry. But why is the August birthstone specifically green? The green color of peridot comes from its chemical composition and the geological conditions under which it forms. By understanding the science behind peridot, we can better appreciate the beauty of August’s green gem.
What is peridot?
Peridot is the gem quality variety of the mineral olivine. Olivine is an iron-magnesium silicate mineral with the chemical formula (Mg,Fe)2SiO4. The magnesium and iron content in olivine is variable, leading to a range in color from yellowish-green to olive-green. Peridot refers to the green, transparent gem quality stones.
Peridot forms deep inside the Earth’s mantle, at depths ranging from 50-150 km. The high levels of magnesium, iron, and silica in the mantle allow peridot crystals to grow. The iron content is what gives peridot its iconic green color. Typically, peridot has an iron content of 10-20%. Higher iron content leads to a darker green color.
How does peridot form?
Peridot crystals form in magma from the mantle. Mantle derived magmas are brought to the surface of the Earth by volcanic activity. As the magma or lava solidifies, it traps crystals of peridot that formed underground. Most peridot is recovered from rocks such as basalts.
Geological processes are required to transport peridot crystals from their underground magmatic origins to locations near the surface where they can be mined. Here are some of the key geological mechanisms:
Xenoliths
Xenoliths are fragments of mantle rock that get picked up in magma and brought to the surface in volcanic eruptions. These fragments break off from vein walls or the surrounding mantle rock. They provide a vehicle to transport deep-formed peridot crystals upwards.
Kimberlite pipes
Kimberlite pipes are formations that extend deep within the mantle, providing a conduit for magma to rise rapidly. This fast upward movement can bring intact xenoliths to the surface containing peridot. Kimberlite pipes are also the main source of diamond crystals.
Tectonic processes
The movement of the Earth’s tectonic plates has brought sections of the upper mantle closer to the surface over geological time. Parts of the oceanic lithosphere can be thrust on top of continental crust in collision zones. This process, called obduction, can create peridot-bearing outcrops of mantle material.
Where is peridot found?
Important deposits of peridot have been found in the following areas:
Hawaii
The volcanic islands of Hawaii provide prime conditions for peridot formation and exposure. Magmas from the mantle hotspot have brought peridot to the surface. Weathering of basalts creates a gem-rich soil called volcanic ash. The famous green sand beaches of Hawaii contain abundant peridot eroded from such deposits.
Arizona
Arizona is one of the most significant sources of peridot worldwide. The state has large ultramafic volcanic complexes with abundant peridot crystals. These form from deep mantle derived magmas. Famous Arizona peridot localities include the San Carlos Reservation and Peridot Mesa.
Norway
In Norway, peridot is recovered from xenoliths in basalts. The xenoliths originated from a depth of around 45 km in the mantle below the basalts. High-grade peridot crystals are released when the basalts and xenoliths weather and erode.
Pakistan
In Pakistan, peridot mining takes place in the Kohistan district. It occurs in ultramafic rocks related to the collision zone between the Indian and Asian plates. Compression and uplift has brought the peridot-bearing rocks closer to the surface.
China, Burma, and Vietnam
Other deposits in Asia derive from regional tectonics exposing mantle material. The Phan Si Pan mountain range between China and Vietnam contains outcrops of mantle rock with significant peridot crystals.
How is the green color produced?
As described above, peridot’s green color comes from its iron content. Specifically, it is caused by electronic transitions involving iron atoms in the crystal structure of peridot.
In peridot’s crystal structure, silicon and oxygen form a tetrahedral framework. Iron, magnesium, and other cations fill the spaces in this framework. The iron atoms can exist in either a ferrous Fe2+ or ferric Fe3+ oxidation state.
When electrons in the iron atoms get exposed to light energy, they can jump up to higher energy levels. The electrons then fall back down to their original states, releasing green photons of light. This process is referred to as an electronic transition.
The intensity of the green color depends on the iron content. Higher iron leads to darker green hues. Trace elements such as nickel can also influence the color.
How are peridot crystals formed?
Peridot crystals grow deep underground in magma chambers beneath active volcanoes. The growth process follows these general steps:
Magma formation
Partial melting of mantle rocks generates magma containing dissolved silica, magnesium, iron, and other elements. This hot magma rises towards the surface due to convection.
Nucleation
As the magma cools, olivine crystals start to form. The initial nuclei are microscopic. High silica magmas allow hopper and skeletal crystal growth.
Crystal growth
With further cooling, the tiny peridot crystals expand outwards by deposition of atoms on the surface. This yields larger, well-formed crystals with defined faces and points.
Transport upwards
Continued convection and tectonic forces transport the peridot crystals upwards in magma. Xenoliths can also encapsulate and carry crystals upwards rapidly.
Eruption and exposure
Once carried to the surface, the peridot crystals can be dispersed in volcanic ash or ejected in xenoliths during eruptions. Weathering then frees the peridot crystals from their host rocks.
What are some key properties of peridot?
| Property | Value |
|---|---|
| Chemical formula | (Mg,Fe)2SiO4 |
| Color range | Yellowish-green to olive-green |
| Hardness | 6.5 to 7 on Mohs scale |
| Specific gravity | 3.25 to 3.50 |
| Refractive index | 1.63 to 1.69 |
| Double refraction | 0.036 |
| Luster | Vitreous to oily |
| Transparency | Transparent to translucent |
| Cleavage | Poor |
Some key properties are:
– Intermediate hardness and durability. Suitable for most jewelry use.
– High refractive indices and strong dispersion lead to “fire” (flashes of spectral colors).
– Olivine crystal structure occasionally shows asterism (star-like light effect).
– Small grain size and fractures limit transparency. Stones under 5 carats are common.
– Chemical composition varies based on iron and magnesium content. Darker green with more iron.
How is peridot graded and valued?
Several quality factors affect the value of peridot gemstones:
Color
The best specimens have a rich olive-green to green color. Yellowish-green is less desirable. Dark tones and high saturation are ideal.
Clarity
Inclusions and imperfections are common in peridot. Eye-clean stones are the most valued. Fractures, grains, and growth lines affect transparency.
Cut
Well cut peridot displays excellent brilliance. Custom cuts with precise faceting maximize optical effects. Table and culet should be centered and aligned.
Carat weight
Larger peridot gems are scarce. Most commercial stones range from just 1 to 5 carats. Pieces above 10 carats are exceptionally rare.
Based on these criteria, peridot is classified into commercial, fine, and very fine grades. The finest stones can reach several thousand dollars per carat. However most peridot is moderately priced.
How is peridot jewelry fashioned?
Peridot’s hardness and durability make it suitable for a wide range of jewelry uses including:
Faceted stones
The most common fashioning technique is to cut and facet raw peridot into standard shapes like rounds, ovals, cushions etc. This brings out peridot’s exceptional brilliance and green fire.
Cabochons
Smooth domed cabochons display the thick rich green color of peridot well. Translucent stones often are cut this way to improve appearance.
Beads
Small peridot beads are very popular for necklaces and bracelets. Their bright color catches the eye. Bead-set peridot also is used to accent other gems.
Antique looks
Lime green peridot combined with engraved filigree or milgraining recreates Victorian era styles. These romantic looks remain fashionable today.
Augmentation stones
Small accent stones around a central diamond or gem is a timeless design. Peridot’s vivid green dazzles as augmentation gems.
Conclusion
The striking green color of peridot has made it a favored gemstone for thousands of years. Its association with the heart of summer in the August birthstone tradition endures today. Peridot’s refreshing color is a perfect match for the green bounty of the season.
Looking deeper, peridot’s green hues originate from its unique geological formation and magnesium-iron chemistry. This combination yields the vivid color that August babies celebrate. Peridot’s beauty illuminates why it has been prized for adornment since ancient times.
