Iolite

An interesting gemstone, a gorgeous deep gemmy blue/purple colour. Iolite is the gem quality variety of Cordierite.

This mineral sometimes displays an optical effect known as pleochroism – viewed from different angles, the stone looks different – it can appear sapphire blue, honey-yellow, or almost clear.

Showing all 5 results

Appearance

Cordierite and Iolite are two varieties of the same material – Iolite being a gem quality variety of Cordierite.

It can occur in several colours and range from transparent to translucent, depending on the quality.

Depending on the quality of the piece and the angle it is viewed at, this material may be light to dark blue, purple-blue, brown, yellow, or grey.

Higher grade pieces can be highly trichroic and will show different colours when rotated.


Uses and History

Cordierite was named after its discoverer, the French geologist Louis Cordier.

There is some speculation, backed by limited historical evidence in the sagas, that Iolite could have been the fabled ‘Viking Sunstone’, used as a navigational aid. The ‘sunstone’ may have also referred to Optical Calcite, a material that was found near navigational tools in a much later ship.

In industry, Cordierite is synthetically manufactured to produce ceramics, particularly for catalytic converters.

The gem grade pieces of Iolite are popular for jewellery, often as an inexpensive substitute for Sapphires.

 

Iolite isn’t often found in mass marketed jewellery.

There is a  reasonably large supply of the material available around the world, but its value a gemstone hasn’t yet been exploited and developed, meaning the prices of rough and cut stones are relatively low.

Iolite can also be relatively fragile due to its cleavage, and it is recommended that jewellers design settings to protect the stone.

 


Locales

Cordierite has a reasonably worldwide distribution, so listing all of the locales is impractical. I’ve listed some of the largest deposits, and some of those with Iolite quality crystals.

Africa:

  • Tranomaro, Amboasary Sud, Anosy, Madagascar
  • Ambatomanoana, Sahanivotry Manandona, Antsirabe II, Vakinankaratra, Madagascar
  • Babati, Manyara Region, Tanzania

Europe:

  • Ebersdorf, Klein-Pöchlarn, Melk District, Austria
  • Horní Bory, Bory, Žďár nad Sázavou District, Vysočina Region, Czech Republic
  • Orijärvi, Kisko, Salo, Finland
  • Bodenmais, Bavaria, Germany
  • Großer Arber, Bayerisch Eisenstein, Regen District, Bavaria, Germany
  • Søndeled, Risør, Aust-Agder, Norway
  • Steinsund, Kragerø, Telemark, Norway

Antarctica:

  • Christmas point pegmatite, Khmara Bay, Casey Bay, Napier Complex, Enderby Land, Antarctica
  • Antarctic Peninsula, Western Antarctica, Antarctica

North America:

  • Slocan Valley, British Columbia, Canada
  • Howard Springs, East Mayacmas Mining District, Lake Co., California, USA
  • Haddam, Middlesex Co., Connecticut, USA
  • Union, Tolland Co., Connecticut, USA
  • Easton, Grafton Co., New Hampshire, USA
  • Richmond, Cheshire Co., New Hampshire, USA
  • Laramie Mountains, Wyoming, USA

Asia:

  • Bidaloti, Koratagere, Tumkur District, Karnataka, India
  • Pavagada, Tumkur District, Karnataka, India
  • Ganguvarpatti, Madurai District, Tamil Nadu, India
  • Panrimalai, Madurai District, Tamil Nadu, India
  • Mogok Township, Pyin-Oo-Lwin District, Mandalay Region, Myanmar
  • Kharlovsk Fe-Ti deposit, Kharlovo, Gornyi Altai, Altai Krai, Russia
  • Elahera District, Central Province, Sri Lanka
  • Ratnapura, Ratnapura District, Sabaragamuwa Province, Sri Lanka
  • Babati District, Manyara Region, Tanzania

South America:

  • Itabira, Minas Gerais, Brazil
  • Virgolândia, Minas Gerais, Brazil
  • Picuí, Borborema mineral province, Paraíba, Brazil

Australia and Oceania:

  • Entia Valley, Harts Range, Central Australia
  • Ambalindum Station, Harts Range, Northern Territory, Australia


Mineralogy

 

Chemistry
Cordierite is a Magnesium Aluminium silicate mineral, which ordinarily contains some Iron. It’s chemical formula is Mg2Al4Si5O18.
Colours and Variations
Light to dark blue, purple-blue, brown, yellow, or grey. Depending on the grade this material may be transparent or translucent.
Streak
White
Luster
Vitreous or greasy
Fracture
Conchoidal to uneven
Crystal habit
Most often grainy or massive. Crystals are rare and often twinned.
Mohs hardness
7 – 7.5
Specific Gravity
2.57 – 2.66
Easiest testing method

One of the easiest ways to distinguish Iolite is with a dichroscope, a jewellers tool used to inspect pleochroism. An examination by eye may be enough to give you an idea, but it can be difficult to distinguish between Iolite and Alexandrite, another colour changing mineral.

However, if you do not have one available, a hardness test will help to distinguish between Iolite (7-7.5), Sapphire (9), and Tanzanite (6-6.5). A streak test and specific gravity test will help to narrow it down from other minerals.


Photos of Iolite

Cordierite-290011

An exceptional gemmy crystal from Minas Gerais, Brazil.

Cordierite-13-cordieritesm

Crystalline Cordierite specimen from New Hampshire.

Cordierite pléochroïsme

Faceted piece from India showing pleochroism.

The left image is in polarised light.

Cordiérite var. iolite 1

A single piece of Iolite at different angles, showing its pleochroism.

Cordiérite var. iolite 2

A single piece of Iolite at different angles, showing its pleochroism.

Cordiérite var. iolite 3

A single piece of Iolite at different angles, showing its pleochroism.


Hazards and Warnings

You’ll find many lists online that say Cordierite/Iolite is toxic because of the Aluminium it contains. While you shouldn’t be putting this material into drinking water, I would point out that a lot of campsite crockery is solid Aluminium. Unless you are eating or inhaling powdered aluminium, the risk of metal toxicity is minimal.

However, almost all rocks, minerals (and, frankly, almost all other substances on earth) can produce toxic dust when cutting, which can cause serious respiratory conditions including silicosis.

When cutting or polishing rocks, minerals, shells, etc, all work should be done wet to minimise the dust, and a suitable respirator or extraction system should be used.


Translations and Synonyms

Arabic:

  • إيوليت
  • كورديريت

Hindi:

Portuguese:

  • cordierita

Bengali:

Indonesian:

Punjabi:

English:

  • cordierite
  • iolite
  • water sapphire

Italian:

Russian:

  • ийолит
  • кордиеритовые

French:

  • cordiérite

Japanese:

  • アイオライト
  • コージェライト

Spanish:

  • cordierita
  • iolita

German:

  • Iolith
  • Cordierit

Korean:

  • 근 청석
  • 아이 올 라이트

Thai:

Gujurati:

Mandarin and Traditional Chinese:

  • 堇青石

Urdu:


Further Reading / External Links