Peridot, The ancient green gemstone from deep within the Earth and the stars.

A little late but still at it for the birthstone of the month blog.

August is/was the best month to be out in nature here in Canada. Hot summer days with long setting suns, cool nights and a lack of bugs in some locations make it the perfect time for camping. Peridot, a lime green gemstone formed deep within the earth, is the birthstone for August. This gemstone is also among being one of the oldest gemstones to be used in adornment and jewellery. The first occurrences of peridot were recorded around 1500 BCE in Egypt. During that time period, the stone came solely from a small island off the coast of Egypt in the Red sea. Today this island is known as St. Johns island or Zabargad Island, but it also has the name “Snake Island” and “Island of Death”. Pharaohs would send workers to the island to harvest the stone day and night for the Pharaoh’s treasury. Interestingly, the way the stone reacts to light in the darkness to give it a “glow” helped the collectors find the stones by firelight. This is how peridot got the named “gem of the sun” and how it got to be Egpyt’s current national stone. 

The passage below is from a great article about the island written by Edward Gübelin describing his 1980 trip to the island. I’ll add the reference below for you to read if you are interested. 

“Diodorus Siculus writes in the first century before Christ: “The Egyptians kept the island under constant watch and anyone who tried to approach the treasure island without permission-let alone to attempt to land and steal the peridots-was threatened with death.” Thus, this island in the Red Sea became one of the most closely guarded regions of the ancient world, and its treasure was held secret for centuries, virtually hidden from the Western world from biblical times until the onset of the Baroque period in the 17th century.” 

I don’t know about you, but this stuff is just cool… Like Indian Jones finding the golden Idol, we have hidden treasures like this all over the world.    

Today Zabargad stands alone and quiet, a ghost of its original glory when it was the only known occurrence of gem-quality peridot. Though it still hosts gem-quality peridot, there have been many other discoveries that have taken the lead in peridot production.  Kyaukpon, in Burma hosts a large collection of good stones. San Carlos, in Arizona, takes the lead as the second-largest producer of peridot followed up by a deposit at Sondmore in Norway, which produces peridots that are slightly lighter in colour and very brilliant when faceted. In addition, some small fragments of peridot can be found by the volcanoes of Hawaii. I even have some I want to try to cut one day. 

Now let’s get dive deeper into the mineral itself and why this is a gem to the earth and the stars. First, how does one classify a Peridot

Peridots Classification: 

1. Orthorhombic Crystal system 
2. Silicate composition 
   1. Nesosilicates Sub-class
3. Olivine Group/family -(Fe,Mg,CaSiO₄)
4. Mineral – Forsterite (MgSiO₄) and Fayalite (FeSiO₄)
5. Variety – Peridot

Peridot is a beautiful stone on its own but did you know it’s a variety of the most abundant mineral within the lithosphere, Earth’s crust. Olivine is the mineral name for peridot (like how corundum is the mineral name for sapphire and ruby; beryl for emerald and morganite) and it is made from silicon, oxygen, magnesium and iron. Just so happens these elements are the most abundant elements in the earth’s mantle and the lithosphere. Now, this does not mean there are large green gems floating around below us, there is more at play here. 

Without getting too technical, olivine is one of the first minerals to crystallize from magma or the “melt”. In rifting zones like the mid-Atlantic ridge and hotspots like Hawaii, there is a lot of melting occurring in the chambers deep below the surface. As this melt rises from the base to the fissures that lead to the surface, olivine will begin to crystallize and fall like REALLY slow-motion molten snow within the chamber. This “olivine snow” then collects along the base of the chamber to form a rock canned peridotite or dunite. This dunite is was actually sits along the base of oceanic crust as it’s formed at the rifting zones in the deep ocean. While not the prettiest of peridots, it’s still interesting to think some of our continents sit upon a layer of gemstones.    

Another really cool tibit of information about this gemstone is that peridot also comes from space. Olivine is a mineral that has formed boht on earth and in space. Stoney and stoney-iron meterorites contain numerous minerals within them, some have gem quality peridots within them. Now these stones come at a preioum price and are smaller than the earth born peridots but still, how cool is it to have a gemstone from the depth of space.  

Olivine is similar to garnet where it is actually a solid solution mix of different elements with a basic chemical structure. These minerals are forsterite and fayalite the magnesium and iron end members of the olivine mineral family. The green colour comes from the iron, so a deeper green means higher iron content while the pale-yellowish peridot is lower in iron. Too much iron results in the mineral becoming black/green and no longer transparent, so there is a sweet spot of about  12-15% iron to magnesium for the best colour. Interestingly unlife corundom and beryl with all of its colourful varieties, peridot only comes in green, gemmologists classify these stones as idiochromotic. The colour is not due to impurities like in sapphires/rubies and emeralds/morganite but rather due the the elements within is chemical structure, malachite and turquoise are other exmaples.     

Now in the past, peridot would have been a stone worthy of imitating or making synthetics. However, just like amethyst, when a stone falls out of favour or they found a massive deposit to flood the market, its value drops. An unfortunate effect of supply and demand. That being said there are still ways (and reasons) to make sure your peridot is a natural stone. The most likely imitator for peridot will again be cubic zirconia (CZ) or glass. The big difference between CZ and peridot is that CZ is cubic and peridot isn’t. Basically, anything mineral not in the cubic crystal system will bend light into two rays as light passes through the crystal. This is due to the atomic structure of the mineral and that structure can cause light to split varying degrees. Some minerals cause such a large split you get the visual effect of “doubling”. You can see this doubling in peridot if you have a magnifying lens. CZ and glass will never have doubling as light is not split into two rays in cubic minerals. Calcite is another mineral that causes doubling, a double image through the stone if you can remember back to science class as a kid learning physics. Another distinguishing feature is the presence of inclusions, peridot will have inclusions whereas CZ will ALWAYS be clean and glass with have spherical bubbles and/or glass swirls. If you ever question your stones, the best option for you is to have a certified gemmologist look at your stones for you. 

Peridot is a vibrant youthful green and great set either as an accent with other greenstones or front and center. A great pick of an ancient stone for the month of August as we keep some of that ancestry in the modern times. Stay tuned next the sapphire of September

Peter