Gemstone profile and identification guide
Iolite
Iolite is the gem variety of cordierite, known for pronounced pleochroism from blue-violet to pale yellow-grey.
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Overview
Iolite is the gem variety of cordierite, known for pronounced pleochroism from blue-violet to pale yellow-grey.
Did you know? Iolite has been linked in popular accounts to historical navigation lore because of polarization effects.
Use this profile as a collector reference and educational guide for quick on-page comparison, then validate with the interactive identification system.
Identification
Name: Iolite
Scientific classification: Silicate (Cordierite)
Hardness: 7.25 Mohs
Transparency: transparent
Lustre: vitreous
Crystal system: Orthorhombic
Primary colour: Violet Blue
Secondary colour: Blue
Colour family: Blues
Strong trichroic pleochroism is the most diagnostic visual feature, supported by RI and birefringence testing.
Verification Status
Status: verified-core-fields
Reference priority: GIA, Gem-A, Mindat, International Gem Society
Core identity, locality, and physical-property fields reviewed against standard gemological references.
Values are given as practical gemological ranges where appropriate.
Physical Properties
Hardness category: hard
Hardness value: 7.25 Mohs
Chemical composition: Magnesium aluminum silicate [Mg2Al4Si5O18]
Crystal system: Orthorhombic
Common locations: India, Sri Lanka, Madagascar, Tanzania, Myanmar
Learn more: Gem-A gemstone education resources.
Colour Variations
Transparent violet-blue to grey-blue cordierite with strong pleochroism.
Optical effects: Pleochroism
Detailed Gemology
Mineral species: Cordierite (iolite variety)
Mineral group: Cyclosilicate group
Chemical formula: Magnesium aluminum silicate [Mg2Al4Si5O18]
Mohs hardness: 7.25
Crystal system: Orthorhombic
Lustre: vitreous
Transparency: transparent
Streak: White
Cleavage: Poor to distinct
Fracture: Conchoidal to uneven
Specific gravity: 2.58-2.66
Refractive index: 1.542-1.578
Optical phenomena: Strong pleochroism
Colour range: Violet blue, blue, grey blue
Common inclusions: Needles, zircon halos, fluid fingerprints
Geological environment: Regional metamorphic terranes and some pegmatites
Principal mining regions: India, Sri Lanka, Madagascar, Tanzania, Myanmar
Important commercial sources: Commercial iolite is supplied primarily by India, Sri Lanka, and Madagascar
Historical significance: Used as an affordable blue faceted gemstone in modern jewellery.
Common treatments: Generally untreated; occasional fracture filling is possible.
Synthetic or imitation forms: Blue glass and synthetic spinel may be used as imitations.
Common look-alikes: Tanzanite, sapphire, blue spinel
Jewellery uses: Used as a faceted gemstone and occasionally as calibrated jewellery stones.
Care recommendations: Reasonably durable but cleavage and brittleness require care; avoid hard knocks and ultrasonic cleaning.
Cross-check profile data with GIA Gem Encyclopedia and Mindat entry for Iolite.
Formation
Forms in high-grade metamorphic rocks and some granitic pegmatitic settings.
Origin
Commercial iolite is supplied primarily by India, Sri Lanka, and Madagascar
Uses
Best for: Used as a faceted gemstone and occasionally as calibrated jewellery stones.
Ideal uses: Earrings, pendants, and protected ring settings.
Care
Reasonably durable but cleavage and brittleness require care; avoid hard knocks and ultrasonic cleaning.
Need another candidate? Use advanced gemstone search or return to the identification guide.
Related gemstones by colour
Related gemstones by hardness
Commonly confused gemstones
Iolite FAQ
How can I identify Iolite?
Compare hardness (7.25 Mohs), crystal system (Orthorhombic), colour profile, and lustre (vitreous) with your sample. Then cross-check related gemstones listed above.
What colour range does Iolite appear in?
Transparent violet-blue to grey-blue cordierite with strong pleochroism.
Where is Iolite commonly used?
Earrings, pendants, and protected ring settings.. For maintenance, follow the care guidance in this profile.
Scientific References
GIA: general gemstone identity (reviewed) - Batch 01 factual verification pass.
Gem-A: gemological terminology and testing references (reviewed) - Batch 01 factual verification pass.
Mindat: mineral species and principal localities (reviewed) - Batch 01 factual verification pass.
International Gem Society: gem trade context and treatment overview (reviewed) - Batch 01 factual verification pass.