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Variety · Silicate

Iolite

(Mg,Fe)₂Al₄Si₅O₁₈

Violet-blue cordierite — strongly pleochroic, once used to navigate by sun.

Hardness7–7.5
Crystal systemOrthorhombic
ClassSilicate
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Iolite specimen

pleochroism of Cordierite - Same specimen. Left image in polarized light. Locality : Coimbatore District, Chennai (Madras), Tamil Nadu, India Size: 1.38 x 1.01 — Photo: Didier Descouens · CC BY-SA 3.0

Iolite

Also known as: Water sapphire.

The gem name for cordierite — a magnesium-iron ring silicate famous for its dramatic pleochroism, shifting blue to gray along different axes.

Intro adapted from Wikipedia (CC BY-SA).

Iolite is a mineral variety of Cordierite, with the chemical formula (Mg,Fe)₂Al₄Si₅O₁₈.

It crystallizes in the orthorhombic crystal system.

Key properties: Mohs hardness 7–7.5; typically blue or purple.

Collections: Gemstones.

Index entry. This entry combines official IMA/RRUFF species data with published physical-property data and a Wikipedia introduction — a full written entry hasn’t been published yet. Browse more I minerals or help write this entry.