Iolite Gem quality of Cordierite

What is Cordierite?

Cordierite is a silicate mineral that is found in metamorphic and igneous rocks. It is typically blue to violet in colour and is one of the most strongly pleochroic minerals. Cordierite has a chemical composition of (Mg,Fe)2Al4Si5O18 and forms a solid solution series with sekaninaite, which has a chemical composition of (Fe,Mg)2Al4Si5O18.
"Cordierite" is a name used by geologists. When the mineral is transparent and of gem quality, it is known as "iolite" in the gem and jewellery trade. Two older names for the mineral are "dichroite" and "water sapphire." The name dichroite means "two colour rock," inspired by cordierite's pleochroic property. The name water sapphire is also related to pleochroism. It was used because a specimen could have the colour of a sapphire when viewed from one direction, but if the stone was rotated it could appear to be as clear as water.

Geologic Occurrence of Cordierite

Most cordierite forms during regional metamorphism of shales and other argillaceous rocks. When formed under these conditions, it is found in schist and gneiss. Less often, it forms during contact metamorphism and is found in hornfels. Cordierite is also found as an accessory mineral in granitic igneous rocks and in pegmatites. When crystals of cordierite have the opportunity to grow without obstructions, they can form short prismatic crystals with a rectangular cross-section.
In metamorphic rocks, cordierite is often found associated with sillimanite, kyanite, andalusite, and spinel. Most gem-quality iolite is produced from placer deposits, where it occurs in association with other gems even though its specific gravity is not high enough to cause a concentration. When exposed to weathering, cordierite alters to mica and chlorite.

Industrial Uses of Cordierite

Cordierite is a mineral with very few industrial uses. It can be used as an ingredient for making ceramic parts used in catalytic converters. However, synthetic cordierite is used instead because its supply is reliable and its properties are consistent. Many other natural materials are losing their place in industry to synthetic materials for these reasons.

History and Introduction

Iolite is a transparent gem-quality form of cordierite, a magnesium iron aluminum cyclosilicate mineral. Although the mineral has a history that dates back hundreds of years, the actual gemstone 'iolite' is considered to be relatively new and lesser-known. Cordierite was officially named after the French geologist, Pierre Louis Antoine Cordier (1777-1861) in 1813. The first significant and exciting discovery of large transparent, gem-quality deposits of iolite was made in 1996 in Palmer Creek, Wyoming (USA) by the American geologist, W. Dan Hausal. The world's largest iolite crystal was discovered south of Palmer Creek, in Grizzly Creek, Wyoming. This record-breaking crystal weighs over 24,000 carats.
The name 'iolite' originates from the Greek word 'ios' meaning 'violet'. Iolite's strong pleochroism earned it the misleading trade name of 'water-sapphire', a name now obsolete. From one direction, iolite can appear sapphire-like blue and from another, it can appear as clear as water. Furthermore, from the top view down, it can appear light golden or honey-yellow in colour. 'Dichroite' is another synonym for iolite in reference to its pleochroic ability; 'dichroite' is a Greek word which loosely translates as 'two-coloured rock'. Iolite is also known as 'the Viking stone' because according to Norse legend, Vikings used iolite as a polarizing filter to help them find the sun on cloudy days. It is believed that the Vikings discovered iolite deposits throughout Norway and Greenland.

Known as "Iolite" by Jewellers

When transparent and of high clarity, cordierite is used as a gemstone. It is known as "iolite" in the gem and jewellery industry. Iolite is a blue pleochroic gem that has an appearance similar to sapphire and tanzanite. It can serve as an alternative stone to either of these gems and is much lower in price. Unlike sapphire and tanzanite, iolite in the gem market is not known to receive heat, irradiation, or other treatments to improve its colour. That is appealing to many people.
Iolite is a challenging material to facet because of its extreme pleochroism. The cutter must examine the stone carefully and have its axis of top-quality colour oriented perpendicular to the plane of the gem's table. A gem of good colour can only be obtained if these cutting rules are followed.
Faceted iolite gems weighing more than five carats are rare. Most stones are two carats or smaller. These small stones often have the best colour because iolite often has a dark tone.
Iolite has a Mohs hardness of 7 to 7.5, which is durable enough for many gem uses. Its main physical disadvantage is its distinct cleavage in one direction. This makes it vulnerable to breakage when used in rings or other items that could encounter rough use.
Iolite is almost never seen in mass-merchant jewellery. It is a gem that is unknown to the average consumer because it is not being marketed. Jewellers do not order it or market it because they are not confident that an abundant supply of quality material will be available to support them. This is surprising because significant iolite resources exist in many countries. Its value in the gem trade has not been developed and thus its price is low.

Identifying Iolite 

Gem-quality iolite can vary in colour from sapphire blue to violet-like blue and from light-blue to yellowish-gray. Its strong pleochroic properties can often be used to help identify and distinguish iolite amongst other similar coloured gemstones. Iolite can sometimes be mistaken for sapphire and tanzanite, but it is softer than sapphire, and harder than tanzanite. Other gems which may also cause confusion include spinel and garnet, but both spinel and garnet are singly refractive, which means they lack iolite's trichroism.

Pleochroism in Cordierite (Iolite)

Pleochroic materials appear to be different colours when viewed from different directions. When viewed from the direction that produces its most attractive colour, most cordierite is a distinct blue to violet in colour. It is one of the most strongly pleochroic minerals. Specimens that produce a strong violet colour can be rotated to produce light violet or dark yellow hues. Specimens that produce a strong blue colour can be rotated to produce yellow or colourless hues.
People who facet iolite must study the stone to determine its direction of best colour. Then they must facet the stone so that the direction of best colour observation is at right angles to the table of the stone. That will produce the best possible colour in the finished gem. See the video on this page for a demonstration of pleochroism in iolite.

Properties of Iolite

Chemical FormulaMg2Al4Si5O18
ColourBlue, Purple, Gray
Hardness7 - 7.5
Crystal SystemOrthorhombic
Refractive Index1.55 - 1.62
SG2.6 - 2.7
TransparencyTransparent to translucent
Double Refraction-0.01
Mineral ClassCordierite