Mineral

Iolite (Cordierite)

Cordierite (Magnesium Aluminum Silicate), Mg2Al4Si5O18

AI-generated identification · may be incorrect

Hardness: 7 to 7.5 on the Mohs scale. Color: Blue, violet-blue, or gray; often shows distinct pleochroism (changing color based on angle). Luster: Vitreous (glassy). Crystal Structure: Orthorhombic. Cleavage: Distinct in one direction. Specific Gravity: 2.58 - 2.66.

Hardness: 7 to 7
Color: Blue, violet-blue, or gray
Luster: Vitreous (glassy)

Identified March 28, 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 pelitic rocks (shales and mudstones) or in contact metamorphic zones; also found in some magnesium-rich igneous rocks like norite. Geological age varies but is often associated with ancient metamorphic belts.

Uses & applications

Used primarily as a gemstone for jewelry (rings, pendants) and by mineral collectors. In industry, the synthetic variety is used in catalytic converters for automobile exhaust systems due to its thermal stability.

Geological facts

Also known as the 'Viking's Compass.' It is believed that Viking sailors used thin slices of iolite as light polarizers to locate the sun on overcast days, allowing them to navigate the Atlantic Ocean.

Field identification & locations

Identify in the field by its distinct pleochroism—rotate the stone to see it shift from blue to yellowish-gray. Commonly found in Brazil, India, Sri Lanka, Madagascar, and Tanzania. Collectors should look for transparent specimens with deep violet-blue hues.