mineral

Iolite (Cordierite)

Magnesium Iron Aluminum Silicate (Mg,Fe)2Al4Si5O18

AI-generated identification · may be incorrect

Hardness: 7–7.5 (Mohs scale), Color: Violet-blue to grayish-blue, Luster: Vitreous, Crystal structure: Orthorhombic, Cleavage: Good in one direction, Specific gravity: 2.58–2.66. It is highly pleochroic, appearing different colors from different angles.

Hardness: 7–7

Identified March 19, 2026More mineral →

Explore Iolite (Cordierite) in the encyclopedia →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Formation & geological history

Formed through the regional metamorphism of clay-rich sediments (pelites) or in contact metamorphism scenarios. It typically occurs in high-grade metamorphic rocks like gneiss or schist, or in contaminated igneous rocks like granites.

Uses & applications

Primary use is as a gemstone in jewelry due to its attractive color and pleochroism. It is also used as a material for ceramic kiln furniture and automotive catalytic converter substrates due to its low thermal expansion.

Geological facts

Iolite is famously known as the 'Viking's Compass.' It is believed that Viking navigators used thin slices of iolite as a polarizing filter to locate the sun on overcast days, allowing them to navigate the Atlantic Ocean.

Field identification & locations

Field identification is best done using its strong pleochroism (it may look blue from one angle and clear/yellowish from another). Commonly found in Sri Lanka, Madagascar, Brazil, and India. Collectors value specimens with high transparency and deep blue color.