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I is for Iolite

Updated: Dec 5, 2021

It's been a while but we're back with our next instalment in the A-Z Gemstones Uncovered series: I is for Iolite. Known for its deep violet and blue hues, it's often referred to as a water Sapphire even though there is no connection between the two stones.

So let's get into it and discover the history of Iolite...

Iolite is considered a relatively new and lesser-known gemstone, due to only having this name since 1912 however the history of this mineral extends far beyond this. Before "Iolite" became its common name, it was referred to as Cordierite, named after French geologist Pierre Louis Antoine Cordier in 1813. Whilst Cordierite is the mineral species, Iolite now refers to a gem-quality specimen of this mineral.

Iolite is also referred to as "The Vikings Stone" or the "Viking's Compass" due to Norse legends (793–1066 AD). Vikings supposedly used Iolite as a glare reducer and polarising filters on cloudy days to help sailors navigate whilst out at sea. The Iolite lens helped the Vikings figure out the position of the sun in the sky, making it possible to travel further than ever before. It is believed Leif Ericsson, a Norse explorer from Iceland, used this unique optical lens, which supposedly aided him in reaching the shores of Central America approximately half a millennium before Christopher Columbus. It is also believed that Vikings discovered the Iolite deposits throughout Norway and Greenland.

Whether this legend is fact or fiction is hard to decipher, but modern navigational experiments have proven that Iolite would work to locate the sun on cloudy days making it a strong possibility that there's a hint of truth within these stories.

During the 18th Century, Iolite became a popular stone used in jewellery however, this seemed to wane dramatically until 1996. An American geologist, W. Dan Hausal, discovered the bluish-violet Cordierite in Palmer Creek, Wyoming which has since become an important source for this stone.

What is Iolite then and how is it made?

As mentioned earlier, Iolite is the name used in gemology and the jewellery trade whereas Cordierite is the term used in mineralogy. So how each one is created is intertwined, as the difference in the name just lends itself to the quality of the mineral.

So Iolite/Cordierite is made up of magnesium aluminum silicate and begins to form when the structure of sedimentary rocks such as shale changes due to heat and pressure. This process either happens during regional metamorphism or contact metamorphism. Regional in this sense means the change in structure happens wide-spread over a region/area whereas contact is a lot more confined. When they form under these conditions they are found in schists, gneisses and hornfels which are all variations of metamorphic rocks. When the crystals in Cordierite have room to grow without obstruction, they start to form short prismatic crystals with a rectangular cross-section.

One of Iolite's most unique and interesting characteristics is its pleochroism. Pleochroism is an optical phenomenon that refers to the way a crystal absorbs different wavelengths of light and how they are reflected off the surface of the crystal. A pleochroic gemstone will display different colours when viewed from different angles. Iolite is one of the most pleochroic gemstones used today, varying from dark violet-blue, light purple and blue tones, dull grey, dark yellow. and even colourless Due to this Iolite is also referred to as a dichroic. It is due to this phenomenon that Iolite was also given the name "water Sapphire" due to its dark blue shades resembling a Sapphire.

Iolite also comes in variations such as 'Bloodshot Iolite', where the presence of hexagonal-shaped hematite platelet inclusions gives it the resemblance of glittery confetti. Sometimes inclusions in Iolite even cause chatoyancy (a band of bright lustre otherwise known as cat's eye).

You can also find Iolite merged with other stones such as Sunstone where they have formed in the same metamorphic rock and entwined.