Iolite
Variety of cordierite
Uncommon · Mg₂Al₄Si₅O₁₈
The gem variety of cordierite, displaying extreme trichroism: blue, yellow, and near-colourless from three different directions. Viking navigators may have used thin slices as polarising filters to locate the sun in overcast conditions.
Identity
- Formula
- Mg₂Al₄Si₅O₁₈
- Chemical class
- Silicate
- Also known as
- Cordierite, Water sapphire (archaic), Viking compass stone
Crystallography
- Crystal system
- Orthorhombic
- Habits
- Massive, Prismatic
- Fracture
- Subconchoidal
Physical properties
- Mohs hardness
- 7.5
- Density
- 2.61 g/cm³
- Luster
- Vitreous
- Colors
- blue, violet, gray, yellow
- Streak
- white
- Transparency
- Transparent, Translucent
Geology
- Origins
- Metamorphic
- Host rocks
- Granite, Schist and gneiss
- Associated minerals
- Garnet, Sillimanite, andalusite, biotite
- Geography
- India, Sri Lanka, Madagascar, Tanzania, Brazil
Classification
- Rarity
- Uncommon
- Gem quality
- Yes
Arcane properties
- Affinity
- Water, Air
- Effects
- Scrying
- Planar origin
- Material Plane
Sources
- Iolite. Mindat.org (Hudson Institute of Mineralogy).
- Iolite. Gemological Institute of America, Gem Encyclopedia.
- Iolite Description. Gemological Institute of America, Gem Encyclopedia.
- J. W. Anthony, R. A. Bideaux, K. W. Bladh, M. C. Nichols, Cordierite. Mineral Data Publishing, Handbook of Mineralogy, 2001.
- Cordierite. Mindat.org (Hudson Institute of Mineralogy).
- M. Pasero and others, IMA Commission on New Minerals, Nomenclature and Classification, The New IMA List of Minerals, updated September 2026. International Mineralogical Association (CNMNC), 2026.
Browse the Crystal Encyclopedia — MGS Toolbox