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The Gemstone Iolite

Iolite is the gem variety of the mineral Cordierite. It describes the transparent to translucent form of Cordierite, and it has recently become a popular gemstone. Iolite can be light to deep blue, and usually has a purplish tinge to it. Deeper colored stones are more valuable. Iolite is very affordable and durable, and can be used as a substitute for more expensive blue gemstones such as Sapphire. It is named from the Greek "violet", alluding to the purplish tinge often exhibited by this gemstone.

Color

Blue, Purple, Gray

Hardness

7 - 7.5

Chemical Formula

Mg₂Al₄Si₅O₁₈

Mineral Class

Cordierite

Additional Properties

Crystal System
Orthorhombic

Refractive Index
1.55 - 1.62

Double Refraction
-0.01

Transparency
Transparent to translucent

SG
2.6 - 2.7

Luster
Vitreous

Cleavage 3,1

All About

Iolite is strongly pleochroic, and will exhibit a different color when viewed at different angles. In fact, it is the most pleochroic gemstone, with this effect being clearly noticeable on all transparent Iolite gemstones. A blue or violet-blue Iolite gemstone will become a duller gray or grayish-yellow when viewed at a different angle. For this reason, care must be taken by gemstone cutters to facet Iolite gemstones so that their colors are oriented with the intense blue or violet showing from the top.