The Perfection of Peridot, part 1: A Glorious Green Gem

A stone with enchanting colouration, peridot is one of the first gemstones to have been distinctly mentioned in early recorded human history, with the use of peridot gems reaching at least as far back as the days of ancient Egypt. Peridot continues to play an important role in bespoke jewellery designs today, and it occupies a space as an official contemporary birthstone for the month of August in many Western nations, sharing this role in North America with the more recently added spinel birthstone. Although not an overly expensive gem, high quality peridot crystals of large size are rare in nature, making top grade peridot jewels highly sought after.

A faceted peridot gemstone from Pakistan; Image: Skyjems

Peridot itself is not a mineral but rather a name given to olivine that is of suitable quality for use as a gemstone. Olivine is the name for a related group of nesosilicate minerals found in numerous locations throughout the Earth’s crust. In fact, olivines are a key component of the Earth’s composition, and they are some of the only gem minerals that can form within the Earth’s upper mantle; because of where these silicate minerals grow, peridot has the potential to be somewhat self replenishing, as volcanic activity often brings more olivine towards the planet's surface from deep within. In its entirety, the olivine group contains a number of minerals but is most often discussed with reference to a solid solution series that contains variable proportions of iron and magnesium within its structure. The magnesium rich endmember of this olivine series is known as forsterite, and the iron rich endmember is known as fayalite. 

A peridot crystal from the Pyin-Oo-Lyin District of Myanmar’s Mandalay Region; Image: Mindat/ Neal Luppescu

The gemstone peridot is almost exclusively cut from high grade crystals of forsterite olivine, which possess an attractive colour not seen in high purity fayalite olivine. Pure forsterite is colourless and pure fayalite is a dark brown colour, but forsterite crystals which contain a small amount of iron display the bright yellow-green hues for which peridot is known; greater amounts of iron tend to cause the colour of peridot to lean more towards a jaundiced yellow-green, becoming gradually browner as the iron content increases. Forsterite crystals that contain approximately 12-15% iron possess the best colour for use as gemstones, with ferrous iron (Fe2+) contributing more towards the green element of peridot’s hue and ferric iron (Fe3+) contributing more towards the yellow hues of peridot. Because of peridot’s chemistry, it is not an allochromatic gemstone and is instead regarded as idiochromatic, meaning that its colour is derived from its native chemical structure and not from the presence of impurities; in addition to this, forsterite is among the short list of gem minerals that display only a single range of closely related hues. Even though peridot is seen as idiochromatic, it has been proposed that in rare cases peridot gems which display the strongest and most vivid green body colours may contain trace impurities of trivalent chromium (Cr3+) that contribute to their appearance indirectly by enhancing the chromophoric activity of iron.

A high purity crystal of fayalite olivine from Sugarloaf Mountain in California, USA; Image: Mindat/ Gianfranco Ciccolini

Understanding the terminology which surrounds peridot is best achieved by knowing more about the history of its name. In the second part of this series, learn about the nuanced series of events which lead to the acceptance of the word “peridot” as a name for high quality forsterite gemstones.

© Yaĝé Enigmus