Iolite

noun

1. A silicate of magnesium, aluminium, and iron, a mineral of a violet-blue color with a shade of purple or black.
2. A silicate of alumina, iron, and magnesia, having a bright blue color and vitreous luster; cordierite. It is remarkable for its dichroism, and is also called dichroite. A variety called Caribbean iolite has a lighter blue color.
3. The clear variety of cordierite.