Iolite is the gem form of the mineral cordierite, a magnesium aluminium silicate with a beautiful violet-blue colour. Its party trick is pleochroism: look through the stone one way and it is rich blue-violet, turn it and it can look pale grey or even honey-coloured. Legend has it Viking navigators used thin slices as a polarising filter to find the sun on cloudy days, earning it the nickname Viking's compass. Main sources include India, Sri Lanka, Madagascar, Tanzania and Brazil.
At 7 to 7.5 on the Mohs scale, iolite is hard enough for regular wear, though it is somewhat brittle and dislikes sharp knocks, so protective settings are smart for rings. It is virtually never treated, which is refreshing: the colour is all natural. Cutters must orient each stone carefully so the best blue faces up, and good cutting makes an enormous difference. It offers a tanzanite-like look at a friendlier price. Clean it with warm, soapy water and skip the ultrasonic.
A natural, untreated alternative to tanzanite's violet-blue. Kyanite matches its deep blue colour too.
