Iolite

Iolite is gem-quality cordierite — a magnesium-iron-aluminum silicate. Its name comes from the Greek ios (violet), and that violet-blue is the whole appeal: a soft, smoky blue that reads like a gentler sapphire or a more affordable tanzanite. For a long time the trade called it "water sapphire," which tells you exactly what buyers reached for it to imitate.

At roughly 7 to 7.5 on the Mohs scale it is hard enough for everyday wear, but iolite has distinct cleavage — it can split along a plane if struck hard — so it is happiest in earrings, pendants, and occasional-wear rings rather than a daily knockabout band.

The dichroism — its defining trick

Iolite is strongly pleochroic: it shows different colors depending on the viewing axis. Turn the same crystal and it shifts from a saturated violet-blue to a washed-out, almost colorless gray-yellow.

This is why cut matters more for iolite than for most gems: the value lives in the orientation. Buy a stone that is vivid face-up, not just from the side.

The Viking's compass

Iolite's most-told story is navigational. Norse sailors are said to have used thin slices of iolite as a polarizing filter — sighting through it to locate the sun's position through cloud and haze, the way a Polaroid lens cuts glare. Whether or not the Vikings literally carried "sunstones," iolite's extreme pleochroism makes the physics plausible, and the legend stuck.

Where it comes from

Most iolite reaches the market from India, Sri Lanka, Madagascar, Tanzania, and Brazil. It is found in larger clean sizes than tanzanite, and it is not routinely treated — the color you see is the color it grew with, which is rare among blue gems.

Buying iolite

• Prioritize face-up color. Tilt the stone; the violet-blue should be strong looking straight down at the table, not only from the edge.
• No-treatment is the norm. Unlike most blue stones, iolite is typically untreated. Be skeptical of anyone charging a treatment premium.
• Mind the setting. Given the cleavage, protective settings (bezels, or prongs that guard the girdle) suit it better than exposed corners.
• It is an alternative, priced like one. Iolite costs a fraction of fine sapphire or tanzanite — its appeal is the look, not investment value.