Cordierite

Cordierite is a magnesium-aluminium silicate best known to gem lovers as iolite, its transparent blue-to-violet variety. Its most striking feature is strong pleochroism: the same crystal can look blue, grey or nearly colourless depending on the viewing direction.

Identifying cordierite

Cordierite is fairly hard (7–7.5) with a glassy lustre, often grey or blue and frequently included. The dramatic colour change with direction (pleochroism) of gem iolite is diagnostic and earned it the nickname 'water sapphire'. It forms in aluminium-rich metamorphic rocks.

Where it is found

Gem cordierite (iolite) comes from Sri Lanka, India, Madagascar, Myanmar and Tanzania, among others, weathering out into gem gravels. Massive cordierite occurs in metamorphic rocks worldwide.