gemstone

Iolite (Cordierite)

Cordierite (Mg2Al4Si5O18)

Hardness: 7 to 7.5 on the Mohs scale. Color: Deep blue, violet-blue, light blue, to yellowish-grey. Luster: Vitreous to greasy. Crystal System: Orthorhombic. Cleavage: Distinct in one direction, poor in another. Strong pleochroism showing different colors from different viewing angles.

Hardness: 7 to 7
Color: Deep blue, violet-blue, light blue, to yellowish-grey
Luster: Vitreous to greasy

Identified July 23, 2026More gemstone →

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Physical properties

Formation & geological history

Formed primarily during regional or contact metamorphism of aluminous rocks (pelites). It commonly occurs in metamorphic schists, gneisses, and contact metamorphic hornfels, as well as in some granitic pegmatites.

Uses & applications

Used as a gemstone in jewelry, valued as a collector specimen, and historically used as a natural polarizing filter (Viking compass).

Geological facts

Iolite exhibits extreme pleochroism—when viewed from one direction it appears deep blue-violet, from another clear or yellow. Because of this property, ancient Norse sailors allegedly used thin slices of iolite as a solar filter to locate the sun on overcast days.

Field identification & locations

Identify in the field by its distinct violet-blue color combined with strong pleochroism, hardness of 7-7.5, and glassy luster. Prominent localities include India, Sri Lanka, Madagascar, Brazil, Tanzania, and Wyoming (USA).