Iolite

Iolite is the transparent blue-violet gem variety of cordierite, a magnesium aluminium silicate. Its name derives from the Greek for violet. Iolite is famous for very strong pleochroism: a single crystal can look deep blue, pale grey and yellowish from three different directions, which earned it the old name 'water sapphire' and, by tradition, use as a navigation aid.

Properties

Cordierite is orthorhombic, hard (7–7.5) and has a vitreous lustre. Because of the marked pleochroism, cutters must orient stones precisely so the best blue shows face-up. It is most often confused with sapphire and tanzanite.

Occurrence

Iolite forms in metamorphic rocks such as gneiss and in some pegmatites, and accumulates in gem gravels. Sri Lanka, India, Madagascar, Tanzania, Brazil and Myanmar are notable sources.