Gem library
Iolite
The Vikings' compass
Iolite is the gem variety of cordierite, a blue-violet mineral of metamorphic rocks. It has the most spectacular pleochroism in everyday gemmology: depending on the axis, the same stone is deep blue-violet, pale yellow or almost colourless.
- Hardness (Mohs)
- 7 – 7.5
- Density
- 2.57 – 2.66
- Crystal system
- Orthorhombic
- Refractive index
- 1.542 – 1.551
- Cleavage
- Distinct on {010}, imperfect
Mineralogical identity
- Family
- Cordierite — cyclosilicate group
- Species
- Cordierite
- Variety
- Iolite (gem variety)
- Chemical formula
- Mg₂Al₄Si₅O₁₈
- Crystal system
- Orthorhombic
- Classification
- Semi-precious stone
- Rarity
- Common
- Indicative price
- €10 – 80/ct
Etymology
From the Greek ion, «violet», and lithos, «stone»: the violet-coloured stone. Its mineralogical name, cordierite, honours the French geologist Louis Cordier.
Physical & optical properties
- Hardness (Mohs)
- 7 – 7.5
- Density
- 2.57 – 2.66
- Refractive index
- 1.542 – 1.551
- Birefringence
- 0.008 – 0.012
- Cleavage
- Distinct on {010}, imperfect
- Fracture
- Conchoidal to uneven
- Lustre
- Vitreous
- Diaphaneity
- Transparent to translucent
- UV fluorescence
- None
- Pleochroism
- Distinct
- Colours
- blue, violet, indigo, grey-blue
Formation, geology & history
Formation and geology
Cordierite is an index mineral of metamorphism: it appears in gneiss, schist and migmatite subjected to high temperature and moderate pressure, and its mere presence tells a geologist about a rock's thermal history. It also occurs in contact aureoles around granitic intrusions. Gem-quality crystals are freed by erosion and concentrated in the alluvial deposits of Sri Lanka, India and Madagascar, where they are recovered by sieving like sapphires. Its colour comes from iron, in a ring-silicate structure close to that of beryl.
Extreme pleochroism
No affordable gem offers such marked dichroism. Along its three crystallographic axes, iolite shows intense blue-violet, very pale blue and an almost colourless yellow-brown. This is both its signature and the cutter's nightmare: an orientation error of a few degrees produces a grey, dead stone instead of the desired blue. The table must be cut perpendicular to the axis of deepest blue, whatever the yield in weight. That constraint explains why well-coloured iolites above five carats remain relatively uncommon, even though the mineral itself is abundant.
The navigators' sunstone
A famous hypothesis, advanced in the 1960s and widely repeated, holds that the Vikings used thin plates of iolite as a polarising filter to locate the sun under overcast skies and navigate the open sea. The idea is appealing and physically plausible: cordierite's pleochroism does make it a natural polariser. Honesty requires a caveat, though: no iolite plate has ever been found in a Viking archaeological context, and the most-discussed archaeological candidates for the sólarsteinn of the sagas are Iceland spar calcite or tourmaline. The fine story remains a hypothesis.
How to recognise it
- The pleochroism testTurn the stone under a light: the shift from deep blue-violet to very pale yellow is immediate and striking. No other common blue stone does this with such amplitude.
- Greyish blue, never brilliantIolite leans toward grey and ink. A vivid, luminous blue suggests sapphire, spinel or tourmaline instead. It is precisely this soberness that makes it recognisable.
- LightnessAt 2.6 it is markedly lighter than sapphire (4.0) or tanzanite (3.2). At equal volume, the difference in the hand is noticeable.
Imitations, treatments & confusions
- Iolite sold as tanzanite or sapphireThe commonest case, in that direction: iolite serves as a cheap natural imitation. Its low density, greyish blue and extreme pleochroism expose it without instruments.
- Blue-violet glassUniform colour, round bubbles, no pleochroism at all. Glass cannot reproduce iolite's two faces, which makes the imitation easy to dismiss.
- Dyed quartz and «iolite quartz»Some beads sold under this name are quartz coloured at the surface or by irradiation. Scratch a hidden area discreetly: surface colour comes off, body colour does not.
Deposits & origins
Sri Lanka, India (Tamil Nadu, Andhra Pradesh), Madagascar, Tanzania, Brazil (Minas Gerais), Myanmar, Norway (the historic southern deposits), Greenland, United States (Connecticut, Wyoming)
Care & precautions
- Warm soapy water and a soft brush: no particular difficulty.
- Avoid ultrasonic cleaning: distinct cleavage in one direction makes iolite sensitive to vibration, especially in thin, elongated stones.
- Indifferent to light and water, the colour does not change over time.
- Store separately; at 7-7.5 it scratches most softer stones and is scratched by corundum.
Traditions & crystal healing
- Chakras
- Third eye · throat
- Zodiac signs
- Sagittarius · Libra · Taurus
- Element
- Air · Water
- Month
- September
Crystal healing makes iolite the stone of inner orientation, leaning on the Viking legend: it is said to help you find your direction in confused periods, discern what matters and support difficult decisions. It is also recommended as a travel stone, slipped into luggage.
The virtues described here belong to tradition and cultural heritage. They are in no way medical advice and never replace a consultation.
Frequently asked questions
Did the Vikings really use iolite?
It is a popular but undemonstrated hypothesis. No iolite plate has been recovered from a Viking site, and the serious archaeological candidates for the sunstone of the sagas are Iceland spar. The physics would work; the archaeology is missing.
Why does my iolite look grey from some angles?
That is its pleochroism, and it is normal. Along one axis it is blue-violet; along another, very pale yellow. A well-cut stone shows the blue in the wearing position, but none shows it from every angle.
Iolite or tanzanite?
Tanzanite is denser, more luminous, usually heated and ten to fifty times more expensive. Iolite is greyer, lighter, and its pleochroism is even more violent. An inexpensive blue-violet is almost always iolite.