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Images courtesy Greg C Grace
Images courtesy Greg C Grace

Colour Investigation: Peridot

Forming deep within the Earth’s mantle, peridot is no stranger to heat and pressure. This green mineral may also be of extra-terrestrial origin, transported within meteorites from the depths of the universe, impacting the Earth in rare, cataclysmic events. STACEY LIM reports.

Peridot is a member of the olivine group of minerals that form an isomorphous series of magnesium-iron silicates. The two end-members of the series are the magnesium-rich forsterite and the iron-rich fayalite. Peridot is the gem-quality variety of olivine and its colour ranges from green, greenish-yellow, yellowish-green, greenish-brown and brown, depending on its chemical composition. 

Pure green gemstones are rare and most peridot exhibits a yellowish undertone. The intensity of peridot’s green hue is determined by the varying amount of iron in its composition; iron influences a yellow-brown tone within the gemstone while traces of chromium and nickel – replacing iron and magnesium – are said to give peridot a bright-green colour.

Pallasite meteorites have expanded the horizons of the study of peridot to the far reaches of the galaxy. Pallasites are one kind of stony-iron meteorite that contain abundant crystalline olivine, sometimes of gem-quality peridot.

"Mining is said to have begun around 300bc, indicating that Cleopatra’s famous emerald collection may in fact have been peridot."

The crystals are generally small and, due to the high iron content of the surrounding iron-nickel matrix, are typically yellowy-brown in colour. Some pallasitic peridot specimens are higher in carat weight and present an attractive green colour favourable for faceted gemstones.

Pallasitic peridot usually contains characteristic inclusions that separate it from terrestrial peridot but the origin of clean gemstones may require chemical analysis.

Affordable and beautiful, peridot can be free of eye-visible inclusions, particularly those in smaller calibrated sizes. Prices rise for gemstones greater than 10mm, particularly those with vivid-green colour. Although larger gemstones can present a high clarity, small and black chromite crystals are characteristic of Earth-formed peridot as are reflective, disc-shaped ‘lily-pad’ inclusions.

Gemstones that are lighter and less intense in colour see a dramatic drop in value regardless of their size, as do those that present eye-visible inclusions. The finest examples of peridot are unearthed in Myanmar and Pakistan with Arizona and China producing more reliable commercial quantities.

Admired since the dawn of civilisation, early records indicate the Ancient Egyptians mined a beautiful green gemstone from the island in the Red Sea called Topazios. The island, now known as St John’s Island, or Zabargad, remains to this day the oldest and longest-known source of gem-quality peridot.

Mining is said to have begun around 300BC, indicating that Cleopatra’s famous emerald collection may in fact have been peridot! A gemstone always associated with light, the Egyptians called peridot the “gem of the sun”, believing it to protect its owner from “terrors of the night”.

Fashioned into a variety of shapes and cuts, faceted gemstones show a doubling of the back facets – an optical property caused by high birefringence and a diagnostic feature of peridot. Carvings, beads and cabochons are also not uncommon although these forms don’t capture peridot’s dispersive nature.

Now the most beautiful samples of peridot come from the border area between Pakistan and Afghanistan and the gemstone is also found in Myanmar, China, the US, Africa, Australia and Vietnam.












ABOUT THE AUTHOR
Stacey Lim

Contributor • Registered GAA Gemmologist & Valuer


Stacey Lim FGAA BA Design, is a qualified gemmologist and gemmology teacher/assistant. She is a jewellery designer, marketing manager and passionate communicator on gemmology. For information on gemstones, visit: gem.org.au









Sunday, 23 September, 2018 04:34pm
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