gemstone

Iolite

Cordierite (Mg2Al3(AlSi5O18))

Hardness of 7 to 7.5 on the Mohs scale, typically blue to violet-blue with strong pleochroism showing different colors from different angles, vitreous luster, poor to distinct cleavage, and specific gravity of 2.53 to 2.66.

Identified August 7, 2026More gemstone →

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Physical properties

Formation & geological history

Forms primarily in aluminum-rich metamorphic rocks such as contact-metamorphosed shales and gneisses, as well as in some igneous granitic pegmatites.

Uses & applications

Used primarily as a gemstone in jewelry and occasionally as a polarizing material in optical instruments due to its strong pleochroism.

Geological facts

Historically known as the 'Viking's Compass', ancient Norse navigators allegedly used thin slices of iolite as a polarizing filter to locate the position of the sun on overcast days.

Field identification & locations

Can be identified in the field by its distinct violet-blue color and strong pleochroism when viewed from different angles. Found in locations such as India, Sri Lanka, Brazil, Madagascar, and the United States.