Iolite Infinity Ring, Size 9

Iolite has a cleavage plane and often carries internal fractures, so a sharp knock or sudden temperature change can split it. Avoid ultrasonic and steam cleaners, and store it apart from harder gems.

Iolite takes its name from the Greek ios, violet. The mineral behind it, cordierite, was described in 1813 from specimens found at Níjar in Almería, Spain, and named for French geologist Louis Cordier. Cut as a gem it has been worn for centuries, and by tradition it marks the twenty-first wedding anniversary.

Iolite is the gem variety of cordierite, a magnesium iron aluminum cyclosilicate with the formula (Mg,Fe)2Al4Si5O18 that crystallizes in the orthorhombic system. Iron gives it its violet-blue color. It is strongly pleochroic, showing violet-blue, pale blue and pale yellow to nearly colorless along its three crystal directions, and it is not typically treated.

• A popular seafaring legend says Viking navigators sighted the sky through thin slices of iolite to find the sun on overcast days, earning it the name Viking's Compass. The idea was proposed by a Danish archaeologist in the 1960s and remains debated; many researchers favor Iceland spar as the sunstone of the sagas.
• Gem traders long sold iolite as water sapphire for its sapphire-like blue, though the two are unrelated minerals and the name is now considered misleading.
• In shamanic ceremony iolite has been carried as a vision stone, held to open the third eye and accompany out-of-body travel.