Iolite

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Iolite
Chemical composition Complex Mg-Al (magnesium aluminum) silicate
Crystal system Orthorhombic
Habit Massive, prisms
Cleavage Distinct, basal
Fracture Sub-conchoidal
Hardness 7.5
Optic nature Biaxial -
Refractive index 1.53 - 1.55
Birefringence 0.008 - 0.012
Dispersion Low, 0.017
Specific gravity 2.58 - 2.74
Lustre Vitreous
Pleochroism Strongly trichroic
Marquise Shaped Faceted Iolite, courtesy of Richard Kocerek.

Iolite is also named "water sapphire", cordierite or dichroite. Iolite has been worn for centuries and derives its name from “Ios” - the Greek word for violet. Due to it’s strong pleochroism, ancient Vikings used iolite for navigation on cloudy days. The gemstone acted as a polarizing filter and allowed the ancient sailors to determine the position of the sun. The soft blue color makes it a very attractive gemstone. Iolite is the 21st anniversary gemstone.