Derived from the Greek meaning “violet”, iolite is the gemological term for the mineral cordierite. While cordierite can be found in other hues, iolite gemstones are almost always blue. Previously known as dichroite, this material is noted for its very strong pleochroism, with a stone’s cutting orientation and/or the angle from which it is being viewed determining whether a deep violetish-blue, yellowish-gray or very pale blue coloration is observed. In fact, iolite's appearance in the latter near-colorless direction has led to it sometimes being known by the misnomer of “water sapphire”.

One interesting historical use for iolite was as a primitive polarizing lens to help sailors locate the position of the sun on an overcast day. This, in combination with the water-like appearance described above, has led to the gemstone becoming inextricably linked with seafaring cultures.