Iolite

Iolite is the gem variety of cordierite, a magnesium aluminium silicate. Its defining property is strong pleochroism: the same stone looks deep violet-blue down one crystal axis and nearly colourless or pale yellow down another, so a cutter must orient the rough carefully or the finished stone shows a washed-out grey. This is the basis of the frequently repeated claim that Norse navigators used iolite as a sunstone to find the position of the sun in overcast skies by polarised light.

The claim is modern and speculative; no medieval source names iolite, the sunstone of the sagas is undescribed, and calcite spar is the material usually proposed by researchers who take the idea seriously. Iolite has also been called water sapphire from its colour. Its metaphysical associations, generally of vision and intuition, are twentieth-century and rest on that colour and on the romantic navigation story.

Iolite is the gem name for cordierite, and the property it is known for needs explaining properly, because it is the basis of every claim made about the stone.

Pleochroism is the absorption of light differently along different crystallographic directions. In a mineral that is optically biaxial, as cordierite is, there are three principal directions and therefore up to three distinct colours, which is called trichroism. Cordierite is the standard textbook example: turned in the hand, a single crystal shows a strong violet-blue down one axis, a much paler yellowish or colourless tone down another, and a pale blue down the third. The change is dramatic and requires no instrument.

The mineralogical basis is straightforward. Light in a crystal resolves into components vibrating along the principal directions, and if the iron in the structure absorbs one component more strongly than another, the colour seen depends on how the crystal is oriented to the eye. Tourmaline, kunzite and andalusite all show it; cordierite shows it most.

This is what a cutter has to manage. A badly oriented iolite is grey or yellowish; a well oriented one is a clean violet-blue. It also caps the size of good cut stones, since the orientation that gives the best colour may not be the orientation that gives the best yield from the rough.

Cordierite is otherwise a common metamorphic mineral, an indicator of low pressure and high temperature conditions, and it is far more often encountered by geologists in a thin section than by anyone in a ring.