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The gem silicates · Indigo ray
Iolite
Iolite is gem-quality cordierite, and its defining property is pleochroism so strong that the same stone looks violet-blue from one direction and almost colourless from another. That property is behind its persistent identification as the solarsteinn of the Icelandic sagas, used to find the sun through overcast sky.
At a glance
Strongly pleochroic, showing violet-blue one way and pale yellow another, and a candidate for the Viking sunstone.
- Composition
- Mg₂Al₄Si₅O₁₈
- Crystal system
- Orthorhombic
- Hardness
- Mohs 7
- Family
- The gem silicates
- Colour ray
- The indigo ray
- Main sources
- India, Sri Lanka, Madagascar, Tanzania, Brazil
- Care
- Hardness 7 but with one cleavage direction. Avoid hard knocks.
Themes
the far look
orientation
seeing through cloud
study
two views
What it actually is
Cordierite, a magnesium iron aluminium silicate, orthorhombic, hardness 7, with one cleavage direction.
Its distinguishing property is pleochroism: it absorbs light differently along its three crystal axes, so it appears violet-blue along one, pale blue-grey along another, and yellowish or nearly colourless along the third. Cutters must orient the rough carefully or the finished stone looks washed out.
That same property is what makes it a candidate for polarised-light navigation. A pleochroic or birefringent crystal held to a cloudy sky and rotated can, in principle, reveal the sun's position by changes in brightness, because scattered skylight is polarised in a pattern centred on the sun.
Iolite is comparatively soft-feeling for hardness 7 and has one cleavage direction, so it chips more readily than quartz does.
Bloodshot iolite contains hematite platelets that produce red flashes.
Where it comes from
India is the main source, with material from Tamil Nadu and Andhra Pradesh, and Sri Lanka, Madagascar, Tanzania and Brazil all produce it. Norway and Greenland yield specimen material.
Cordierite itself is a common metamorphic mineral; gem-quality transparent material is much less common.
The stone was named iolite in the nineteenth century from the Greek for violet, and it was earlier called dichroite, from its two visible colours, and water sapphire, which is a misleading trade name.
Its market presence has always been modest, partly because good clean stones of size are uncommon and partly because the pleochroism makes it difficult to cut well.
What the traditions claimed
The Viking navigation question is the most interesting claim attached to any stone in this catalogue, and it deserves an honest summary.
Icelandic sources mention a solarsteinn, a sun-stone, used to locate the sun in overcast conditions. No such stone has been recovered from a Norse context, so the identification is inference rather than evidence. The three candidates are Iceland spar calcite, which is birefringent, cordierite, which is pleochroic, and tourmaline.
A calcite crystal recovered from the wreck of an Elizabethan ship in the Channel showed the technique was at least known and used centuries later, and experiments have demonstrated that skilled use of such a crystal can locate the sun to within a few degrees under cloud.
Iolite's case rests on its pleochroism being visible without special equipment. It remains unproven, and any page stating flatly that the Vikings used iolite is overreaching.
Beyond that, iolite has almost no traditional lore, and its modern reading about vision and inner sight is recent.
How it is worked with
The orientation reading is the useful one and it comes directly from the stone: iolite is about finding your bearing when the usual reference is hidden.
That is a narrower and better brief than the general intuition assignments, and it suits questions where you know roughly where you are going and cannot currently see the way.
For wear, it is reasonable at hardness 7 but has one cleavage direction, so a protective setting helps and hard knocks should be avoided.
Turn any iolite you are considering in the light before buying. A well-oriented stone shows a good violet-blue face up; a badly cut one looks grey and lifeless from the same angle, and the difference is entirely in the cutting.
Clean in warm soapy water. Avoid ultrasonic cleaners.
Buying it honestly
Iolite is not heavily treated, which is unusual, and there is no standard heating or irradiation practice.
The main issues are substitution and cutting. Tanzanite, blue sapphire, blue spinel and synthetic material are all sold as iolite or in place of it, and iolite is also sold as water sapphire, which is a trade name and not a sapphire of any kind.
The pleochroism is the identifying test that no substitute reproduces at the same strength: turn the stone and the colour should shift markedly between violet-blue and pale yellowish grey.
Glass shows no pleochroism at all and warms quickly in the hand.
Poorly oriented natural stones are the most common disappointment, and that is a cutting failure rather than a fraud.
Handle with care
Hardness 7 with one cleavage direction: it chips under hard knocks. Avoid ultrasonic cleaning. Otherwise safe in water and stable in light.
Fikr yuritish uchun savollar
- Where am I trying to navigate without the reference point I usually use?
- What looks completely different from another position, and have I checked?
- What would I do if I could only see the direction and not the destination?
Follow the thread
Frequently asked questions
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