A Journey into the Blues of Iolite from Tamil Nadu

Gem-A graduate Nishka Vaz FGA shares an abridged version of her stellar Gemmology Diploma project, which focuses on the various colourations of iolites from Tamil Nadu, India.

Few gemstones can boast myriad colours like iolite, one of the most uniquely pleochroic gemstones known. Pleochroism is the name given to the optical effect where a gemstone displays different colours along the different directions in which it is viewed.

One of the pleochroic shades of blue in iolite resembles that of a sapphire so closely that it has earned the title of saphir déau or ‘water sapphire’. When viewed with the naked eye, iolite shows three different pleochroic colours, thus, it is said to be trichroic. The name ‘iolite’ itself is derived from the Greek word ion, which means violet.

An iolite cube cut to display all three pleochroic colours.

As the legend goes, Vikings would carry plates of iolites on board their ships. On cloudy or foggy days with excessive haze and low visibility, it was difficult to mark the exact position of the sun in the sky.

In such instances, navigators would use their iolite plates as a polarizing filter to pinpoint the location of the sun and, using a sundial, determine the geographical north, thus steering the ship back to its correct course; this lead to another common name for iolite: the Viking Compass Stone.

Read more: Understanding the Value of Sapphires

In India, one prime source of these beautiful gemstones are the gem-bearing lithologies of the southern states. Of these, Tamil Nadu has multiple small pockets of gem quality iolites littered around veins found over its vast rolling plains and steep hills. The rocks of southern India converge through a huge network of shear zones and faults to form a lattice of highly metamorphosed rocks.

These rocks guard a variety of different gemstone species. Iolites, rubies, alexandrites, sphenes, sunstones, rock crystals, zircons, aquamarines, and emeralds are just some of the many beautiful gem quality mineral specimens found nestled in these formations.

Iolites from Kiranur, Tamil Nadu.

The Palghat-Cauvery Shear Zone is especially famous for its abundant gemstone mineralization. Iolites, however, may be found sporadically over this large area. Places like Lachmanapatti, Malapatty, Kiranur, Karur, etc., are well known for the presence of gem quality iolite.

Read more: What Should Be in the Ideal Gemmologist's Toolkit?

Obtaining these coveted specimens, however, is not an easy feat. In some cases, they may be mined out of very large pits, or quarried from exposed cliffs. The locals have their own method of extraction: they excavate vertical shafts with a cross sectional area of one square metre extending down to depths of roughly 130 feet. 

When they reach a horizon that appears to be gem-bearing, they then radially extend horizontal tunnels of similar dimensions into the host rock. Multiple shafts and adits eventually form a network of interconnected mining tunnels, not unlike ant colonies.

However, these are only just large enough for a single person to pass though at any given time. If they chance upon a gem-bearing vein lodged between tougher horizons, they only excavate the vein and try to leave the host untouched.

Entrances to iolite mining shafts.

Iolite is a magnesium aluminum silicate, chemically represented as Mg2Al4Si5O18. Iolite is usually found in metamorphic rocks, which are assumed to be formed from the high pressure and temperature alteration of rocks composed of aluminosilicates such as clays, along with an abundance of magnesium rich sediments.

In Tamil Nadu, iolite is found in schists, granulites, and in some cases, even weathered gneisses. They occur along with minerals such as feldspar, sillimanite, biotite, phlogopite, quartz, haematite, diopside, actinolite, etc. which form the matrix of the host rock. Although gem-quality, these iolites do not show proper crystal faces, thus, they are said to have an annhedral habit.

Iolite from Perur, Tamil Nadu.

Tamil Nadu’s reserves being vast, naturally a variation in gem size and quality should be expected. However, a drastic change in size, colour, and overall quality is observed in iolites obtained from across this region. Even accounting for pleochroism, there are iolites found that exhibit colours ranging from pale, washed out blues and violets to deep inky blues that verge on black.

These stones may be so densely included that they almost appear opaque; alternatively, they may be nearly free of inclusions.


Iolite from Arasanatham, Tamil Nadu.

The rare and beautiful bloodshot iolites are also found in Tamil Nadu. They are characterized by an abundance of hexagonal shaped haematite platelet inclusions.

Each platelet behaves as a tiny mirror that reflects light in various directions when randomly oriented. Unlike sunstones, each haematite platelet in a bloodshot iolite reflects a different colour, giving rise to a colourful spangled effect, resembling a colourful glitter confetti.

Spangled effect in bloodshot iolite under 10x magnification.

Even more incredible is the display of chatoyancy and asterism in some of these stones. They may show a faint but clearly visible cat's-eye or star caused by the reflection of light off these haematite inclusions.

A wide range of minerals are found as inclusions within these iolites. Naturally, hexagonal haematite platelets are found abundantly as inclusions in the bloodshot iolites. If these platelets are randomly oriented, they produce a spangled effect.

If they are oriented along a single plane, they produce a cat's-eye effect. If they are oriented along three directions at 120˚, light reflects along a line in each of these orientations to produce an asterisk-shaped reflection pattern of light. This effect is called asterism.

Cat's-eye bloodshot iolite.

An extensive variety of other minerals are also found as inclusions within these iolites. Sillimanite may be seen as long, thin needles which could be oriented in a particular direction, or be randomly dispersed throughout the gem body. Rhombohedral inclusions of calcite are also sometimes observed. Diopside may occur as perfect euhedral dark green to black inclusions.

Read more: Five Things to Consider Before Starting the Gem-A Gemmology Foundation Course

Olive green mineral inclusions from the amphibole group may also be found with a perfectly preserved prismatic habit and deep grooves or striations on their prismatic faces. Well-formed or euhedral inclusions of orange spessartine garnet, or rods of apatite may also occur in the iolites from Tamil Nadu.

Iolite gemstones procured from various localities in Tamil Nadu.

Iolites are fascinating gemstones. The wide range of optical effects that they are capable of displaying is truly captivating – from their unique, visible pleochroism, to the range of shades they can exhibit.

From their sparkly spangled appearance to displaying chatoyancy and asterism, these eccentric gemstones never fail to put on a spectacular display of optical effects. They are incredibly versatile and, in any of their many forms, are certainly a sight to behold.

Interested in becoming a gemmology expert with Gem-A? Read more about our Gemmology Foundation course.

Gem-A also offers a host of one-day workshops on a range of gemmological subjects. Check out our upcoming workshops here.

Cover image: rough iolite specimens. All images are author's own. A bibliography and references are available on request.

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Hidden Treasures: Highlights of Gem-A's Gemstones and Minerals Collection

Locked away in the vaults of Gem-A HQ in London is an assortment of breath-taking treasures that form Gem-A’s Gemstones and Minerals Collection. Here, Gem-A Gemmology Tutor, Pat Daly FGA, offers us a glimpse at some of the more unusual items in the collection.

Decades of collecting, bequests and acquisitions have led to the creation of the Gem-A Gemstones and Minerals Collection, which includes red beryl, banded fluorite, citrine, gypsum, peridot, zoisite and many more specimens that are worthy of conversations in their own right. Now, thanks to fantastic photography by Gem-A’s Henry Mesa, we can share some of these unique gemstones with you.

An example of a coral skeleton. Image by Henry Mesa, Gem-A.

 

Coral Skeleton

This example shows a classic coral skeleton: a branching, porous supporting structure produced by a colony of marine invertebrates.

Read more: Saltwater versus Freshwater Pearls

Precious coral, on the other hand, is a more compact and less porous material with a deeper saturation of colour. The sale of coral, like all organic gem materials, is subject to controls, which are designed to preserve marine faunal diversity and maintain stocks for the future.

A rough jadeite specimen. Image by Henry Mesa, Gem-A.

Jadeite

Also part of the Gem-A Collection is this cobble of rough jadeite which has been polished on one side to reveal the bright lustre modified by a dimpling, which is common on polished jadeite. This has a pleasing variation of white, green and lavender colours and a granular structure.

Read more: Jade and its Importance in China

 Jadeite is a polycrystalline gem material, composed of many small interlocking crystals. This structure gives it great strength and its resistance to breakage means that this gemstone can be made into delicate carvings, which are highly valued in China.

Emerald in Pyrite

This specimen (see cover photo) showcases well-formed crystals of emerald from Muzo, Colombia, in pyrite. Both minerals are formed from hot aqueous fluids circulating at high-levels and under great tectonic pressure in the earth’s crust.

A rough tanzanite crystal. Image by Henry Mesa, Gem-A.

Tanzanite

The mineral zoisite is well-known as an abundant compound in some rock types, but it was of little significance to the gemstone trade until 1967, when tanzanite was found in a relatively small, two kilometre-wide belt in Tanzania. This well-formed crystal of tanzanite now forms part of the Gem-A collection, along with other cut and faceted tanzanites with deeper purple tones.

A box of 19 gemstones from the Anderson Collection, including a 30.82 carat beryl (centre). Image by Henry Mesa, Gem-A.

The Anderson Collection

This historically significant collection of gemstones was once owned by Gem-A founding father, Basil Anderson. In the 1980s, the entire collection of exceptional quality gems was destined for auction and risked being lost to the Association forever.

 

Gems from the Anderson Collection, including a 39ct aquamarine specimen (centre). Image by Henry Mesa, Gem-A.

 

Recognising this, an anonymous donor purchased the entire collection and donated it to Gem-A in 1986. Highlights include a specimen box containing 17 gemstones, including a 30.82 carat beryl, and a second box containing nine gems with a 39 carat aquamarine.

Cover image: Muzo emerald crystals in pyrite. Image by Henry Mesa, Gem-A.

This article was originally published in the Autumn 2018 issue of Gems&Jewellery (Volume 27, No.3).

Are you passionate about gemstones but new to the science of gemmology? Why not try one of our Introduction workshops.

Interested in taking your gemmological education further? Take a look at our Gemmology Foundation course

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American Gemstones: Benitoite from California

In 1907, by the headwaters of California's San Benito River, a new blue gemstone was discovered which was to be named benitoite. Here, Elizabeth A. Gass FGA, Gemstone Advancement and Education Coordinator at Jewelry Television (JTV), takes a closer look at this all-American gemstone. 

The United States is a geographically vast and geologically diverse nation. It comes as no surprise that it would host some of the most sought-after gemstone species in the world. Some of these are wholly unique, while others are some of the finest quality known to man. One mineral, with a rather unique discovery story and geological occurrence, is benitoite. 

Benitoite was discovered in 1907 by James Marshall Couch, a failed melon farmer turned prospector, near the headwaters of the San Benito River in the mountains of the Diablo Range (Wilson, 2008). 


A benitoite crystal in matrix. 

The type locality of benitoite, the New Idria district, San Benito County, California, is also the only location that has produced facet grade benitoite. Out of less than 10 known locations around the world, only three are outside of California (Japan, Australia and Arkansas, USA), and only two mines in the New Idria district, the Benitoite Gem Mine and Junila claim, have produced any facet grade material. Of those two mines only one ever produced faceted material in commercial quantity, the Benitoite Gem Mine. It was also the original mine opened at the site of discovery (LaursRohtert & Gray, 1997).  

The Benitoite Gem Mine operated in some capacity from 1907–2005 under several different names and owners. The site was reclaimed in 2005 and sold (Wilson, 2008). It is currently open as a small pay to dig operation. 

The Race to Name Benitoite

When benitoite was first discovered it was thought to be blue diamonds by Mr. Couch, who showed them to his financier Roderick William Dallas. Dallas immediately sent Couch back up the mountain to stake a claim.  

All the formal paperwork was filed within eight days of the initial discovery, but no one involved knew exactly what kind of blue gemstone they were looking at. Dallas asked Leland Barnes Hawkins, Sr., a mine engineer and friend to look at some of the rough crystals and take them to a jeweller in Los Angeles to determine if they had value. 

Learn More: Red Beryl from Utah

The jeweller declared them to be a form of blue obsidian. Then another jeweller identified them as spinel because of their colour. A faceted sample, labelled as a spinel from California, was sent to a jewellery firm in San Francisco. The head of their diamond department studied the stone and realised that the gem wasn’t anything he had seen before. He purchased the stone for $40 and sent it to a friend, Davis Louderback, who was an Associate Professor of Geology at the University of California, Berkley. 

Benitoite in matrix.

After concluding that the stone was neither spinel nor sapphire, he asked for more material, preferably crystals, to confirm that this was a new mineral. After backtracking the gem to its source, it took Louderback only a few days, with the new samples, to confirm that Couch and company had a new mineral on their hands. From there, a visit to the mine was required so that Louderback could write a formal bulletin on the discovery. 

Word of a possible new gemstone reached Tiffany & Co.’s George F. Kunz, who sent a telegram asking for more information on the discovery. From there the race was on to see if Louderback could complete his research before Kunz wrote his own proposal. Others involved with the discovery pushed Louderback to tirelessly work on his proposal because they wanted their Californian gem to have a “California name”. Louderback succeeded and on July 30, 1907 his report was published, officially naming the new mineral ‘benitoite’ (Wilson, 2008). 

Benitoite's Geological Origins

Benitoite’s discovery could have never happened without the unique geology of the New Idria region. The New Idria region is predominantly composed of a serpentinite body that was tectonically emplaced into the surrounding sedimentary and metamorphic rocks (LaursRohtert & Gray 1997). When two tectonic plates collided, one dipped below the other in a process called subduction. This creates a massive amount of heat and pressure. Because of this the serpentinite rose through the overlying rocks due to its relatively lower density as a plume.  

As it rose it captured some of the overlying rocks, namely those of the Franciscan Complex, which is composed of a wide variety of rocks like basalt, greenstone, chert, limestone, sandstone and blueschist. The main constituents are graywacke sandstone and blueschist, but all the mentioned rock types can be found and are highly folded and faulted in the Franciscan Complex. The serpentinite is composed mainly of chrysotile-lizardite serpentinite and minor antigorite serpentinite (Tsujimori, 2007). With these randomly interspersed pieces of the Franciscan Complex the geochemistry varies widely from one area to the next.  

Rough benitoite (right) and a cut and polished benitoite gemstone (left).

Once these rocks were in place, a later intrusion of syenite (intrusive igneous rock similar to granite but containing less than 5% quartz) in the southern portion of the serpentinite body took place. This caused hydrothermal alteration to occur in localised areas of the serpentinite and its tectonic inclusions (Tsujimori, 2007).  

Learn more: Sweet Home Mine Rhodochrosite from Colorado

The benitoite is solely found in the hydrothermally altered zones of the blueschist and, even then, only when the amphibole and pyroxene show recrystallisation; there is dissolution of albite and pervasive infilling of natrolite in the veins (LaursRohtert & Gray, 1997). Even then, the benitoite usually only forms where the veins narrow or terminate and it is always coated by natrolite, which formed in the later stages of alteration. Geologically this is a very narrow window for the growth and formation of benitoite and other accessory minerals like neptunite. 

Benitoite's Gemmological Properties

Because of benitoite’s rarity in the world and its very specific growth requirements, it should come as no surprise that its crystallographic properties are also unique. It is a barium titanium silicate (BaTiSi3O9) that forms in the trigonal portion of the hexagonal crystal system. The common form that it crystalizes in is a ditrigonal-dipyramidal habit, which is the most complex form of a trigonal crystal (Louderback & Lawson). This form is so rare, in fact, that it took 77 years for a natural occurrence of this morphology to be found after it was predicted in 1830 (LaursRohtert & Gray, 1997). 

Crystals are typically smaller than one centimetre with many of them being highly included. Gemstones are typically very small due to the flat habit of the crystals, with strong colour zoning and inclusions of amphibole and pyroxene.  

Examples of cut and faceted benitoite in a range of hues.

Cut stones are typically smaller than one carat but many stones larger than 4ct have been produced over the years with the largest cut benitoite weighing in at 15.42ct (LaursRohtert & Gray, 1997). Don’t let the relatively small size of benitoite fool you though; it can come in a beautiful range of colours from blue to slightly purplish-blue, or white to colourless, with very few stones being naturally pink.  

Most stones are not treated in any way, but an orange colour can be caused in some stones through heat treatment. Even though the dispersion is sometimes masked by its deep colour, it is in fact higher than that of a diamond. This sets it apart from many other similarly coloured gemstones.  

Learn more: Yogo Sapphires from Montana

The state gemstone of California is as American as a gemstone can be. It comes from unique geological circumstances, has a rare crystalline structure, and has an incredible story of its discovery by a humble melon farmer turned prospector, looking to improve his family’s lot in life, through determination and a lot of luck. Considering all this, benitoite is truly one of the greatest American treasures. 

This blog was originally published as part of the article, ‘One Nation, Many Gemstones’ in Gems&Jewellery Vol. 29, No. 1. 

Discover more about coloured gemstones in Gem Knowledge, part of the Gem-A Gem Hub. Find out more about JTV here. 

Interested in becoming a qualified gemmologist? Take a look at our Gemmology Diploma and Gemmology Foundation courses. 

Main image: An example of benitoite in matrix. Complete bibliography and references available upon request. All images courtesy of JTV. 

 

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