The Perfection of Peridot, part 3: A Joy Among Jewels
Peridot is a wonderful stone to use in bespoke jewellery designs due to both its colour and reasonable availability. Although the rapid weathering of olivine makes large high quality peridot gems over 10 carats rare, stones up to 5 carats are not as scarce, allowing peridot to take the spotlight as the centre stone of a custom ring or pendant just as easily as it may occupy the role of a side stone or accent gem. The moderate hardness and poor cleavage of peridot also give this green treasure a fairly durable constitution, however this durability is not as high as that of beryl, topaz, ruby, or sapphire, meaning that care must be taken while wearing peridot jewellery on the fingers or wrists; everyday jewellery featuring peridot gemstones should be limited to earrings and necklaces.
An attractive green gemstone, peridot shines with a unique brightness. The refractive index and dispersion seen in peridot is much higher than that of most vitreous stones, causing gems to exhibit gorgeous scintillation and colours which shift slightly between adjacent wavelengths of yellow-green light. The birefringence of peridot is also somewhat high, giving the stone’s internal reflections an ethereal appearance. Peridot’s strong light refraction and optic doubling make it readily distinguishable from other vitreous gems of similar colour, such as green tourmaline or pale green beryl. Although known to be a gemstone with strong brilliance, peridot gems typically bear small inclusions due to the conditions under which they form. As a gemstone, peridot is regarded by the by the Gemological Institute of America as having what they call “type 2” clarity; for type 2 clarity stones, gems which bear inclusions visible under 10x magnification are significantly more common than they are for higher clarity “type 1” gems such as aquamarine and tanzanite, with “type 3” clarity gems like emerald and rubellite tourmaline often bearing inclusions visible to the naked eye. The inclusions found in peridot most often are disk shaped inclusions known as “lily pad” or “lotus leaf” inclusions. Small platelets of mica and tiny fluid pockets are also somewhat common inclusions in peridot gems, with other minerals like chromite, serpentine, chlorite, and magnesite also appearing within crystals from time to time. Certain sources of peridot produce gems with highly distinct inclusions, like the mines of Sapat Gali in the Kaghan Valley of Pakistan’s Khyber Pakhtunkhwa Province which have yielded stones containing intriguing distributions of acicular ludwigite crystals. Generally, no treatments are applied to peridot in order to enhance its colour or clarity, however fracture filling of lower quality gems is not completely unheard of.
Peridot quality forsterite has been successfully synthesised under laboratory conditions using melt crystallisation techniques, but such stones are primarily used for research purposes and only appear in jewellery on very rare occasions. Synthetic forsterite bearing trivalent chromium (Cr3+) is sometimes used as part of laser technology, and when cut in gemstones it can be readily separated from natural peridot due to a difference in specific gravity and refractive index; lab grown gems of typical forsterite are optically identical to natural peridot but can still be identified by their inclusions. Although synthetic peridot gemstones are rare, peridot is sometimes simulated with synthetic green corundum, synthetic green spinel, synthetic green quartz, and even green glass.
From deep within the Earth, peridot crystals can travel a great distance before completing their journey as cut gemstones. Learn about global sources of peridot and the origins of olivine formations in the next instalment of this series.
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