silicates
Iolite
- Also known as
- cordieritewater sapphireViking's compass
- Crystal system
- orthorhombic
- Hardness
- 7–7.5 on the Mohs scale
- Lustre
- vitreous
- Colour
- Violet-blue, appearing grey or yellow from other angles
- Where it forms
- In metamorphosed aluminium-rich sediments.
- Chiefly from
- Sri LankaIndiaMadagascarNorway
Why it is that colour
Iron. Its defining feature is extreme pleochroism: the same stone looks deep violet blue down one axis, pale grey down another, and yellowish down a third.
Safety
Safe to handle.
Where it comes from
Sri Lankan, Indian and Madagascan material. A modest trade.
Historical use
Named in 1813 after Pierre Louis Antoine Cordier. Its pleochroism makes it difficult to cut, since the cutter must orient the stone to catch the blue axis.
Recorded as history and attributed where possible. That a stone was used for something establishes that people used it, not that it worked.
What is attributed to it, and since when
The persistent claim that Vikings used iolite as a polarising filter to navigate is popular and poorly evidenced — no iolite has been found in a Norse navigational context, and the better candidate for the solarsteinn is Iceland spar, which genuinely polarises light and has been recovered from a shipwreck. Iolite has no documented traditional attribution.
Described, not endorsed. Dating a claim is not the same as assessing it — but an association that first appears in a book published in 1985 is not an ancient inheritance, whatever the label says. A sincere account of feeling better is compatible with the stone doing nothing — see the placebo effect.
Sold as this, but not
Worth knowing
The Viking sunstone claim attaches itself to three different minerals in popular writing — iolite, sunstone and Iceland spar — and only one of them has any evidence behind it.
The physics needs a mineral that separates or filters polarised light, so that the polarisation pattern of an overcast sky can be read to find the sun. Iceland spar does this strongly, has been tested experimentally, and a crystal was recovered from the 1592 Alderney wreck. Iolite is pleochroic, which is a different phenomenon — it changes colour with viewing direction, not with polarisation angle — and no archaeological example exists.
It is a good illustration of how a story migrates. The claim is repeated confidently about whichever stone is being described, and the actual candidate is a transparent calcite nobody is selling.
Identity checked against IMA-CNMNC master list, June 2025.