What Is Iolite? The Stone Vikings May Have Used to Navigate

Iolite is the gem name for cordierite, a magnesium iron aluminum silicate mineral. It's sometimes nicknamed "water sapphire" for its blue-violet color, but the name is misleading — iolite is chemically and structurally completely unrelated to sapphire (which is a variety of corundum).

Extreme Pleochroism

Iolite is one of the most strongly pleochroic gem minerals known, typically showing three distinctly different colors depending on which of three crystal axes you view it through: a deep blue-violet from one direction, a pale yellow-grey from another, and a light blue from a third. This is more pronounced than in most other pleochroic stones, including kunzite, and cutters must orient the rough carefully to showcase the most attractive blue-violet face-up.

The Viking Navigation Connection

Historical and archaeological evidence — including a fragment of cordierite crystal found aboard a sunken 16th-century English ship — supports the idea that Norse sailors may have used iolite (referred to in sagas as "sunstones") as a natural polarizing filter. Held up to an overcast sky, its strong pleochroism can reveal the sun's position even through cloud cover, by showing a shift in color/darkness as the crystal is rotated relative to the polarization of skylight — a genuinely plausible pre-compass navigation aid, though the extent of its actual historical use remains debated among researchers.

Where It's Mined

India, Madagascar and Sri Lanka are the major current sources of gem-quality iolite.

Quick Facts

Mineral family: cordierite — Hardness: 7–7.5 on the Mohs scale — Extremely strong pleochroism (three colors from three angles) — Historically linked to Norse navigation — Major sources: India, Madagascar, Sri Lanka.