Iolite (Cordierite)
Iolite is the gem variety of cordierite, famous for its extraordinary trichroism — it shows blue-violet, yellow-grey, and nearly colorless from three different viewing directions. Vikings reportedly used thin iolite plates as a navigation tool to locate the sun on overcast days. In the US, Connecticut and Wyoming have produced gem-quality iolite. It offers sapphire-like blue color at a fraction of the price.
Physical Properties
Chemical Formula
Mg₂Al₄Si₅O₁₈
Hardness (Mohs)
7–7.5
Color
Violet-blue, blue-grey, yellow-grey — strong pleochroism
Luster
Vitreous
Crystal System
Orthorhombic
Streak
White
Cleavage
Imperfect
Specific Gravity
2.57–2.66
🔍 Field Identification Tips
Rotate the crystal and watch the color change dramatically — blue-violet in one direction, yellowish in another. This trichroism (three different colors at 90° angles) is diagnostic. Hardness 7–7.5. Vitreous luster. Found in metamorphic rocks. Distinguished from sapphire by lower SG and strong pleochroism.
Varieties & Forms
📍 Where to Find Iolite (Cordierite) in the US
Uses & Applications
Jewelry (sometimes called "water sapphire"), collectors specimens. Vikings may have used it as a polarizing navigation tool.
💰 Collector Value Guide
Rough: $5–$50. Faceted gems: $10–$100/ct. Top violet-blue stones: $100–$300/ct.
Find Iolite (Cordierite) With Our Tools
Use the We Lick Rocks app to identify specimens, find dig sites, and connect with local rockhounds.