Cordierite (Iolite)

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Marketed as iolite, cordierite is a magnesium iron aluminum silicate famous for exceptionally strong trichroism, appearing deep violet-blue, pale yellow, and near-colorless when viewed along its three crystallographic axes. That property earned it the old name water sapphire and a role as a navigational aid, since Vikings may have used it to locate the sun through cloud. It crystallizes in the orthorhombic system and measures 7.0 to 7.5 on the Mohs scale. The finest gem material comes from Sri Lanka, India, and Tanzania, yielding medium to dark violet-blue stones at accessible prices, so this moderately priced semi-precious gem endures as an underappreciated alternative to blue sapphire.

Physical & Optical Properties

Category Semi-Precious Gems

Mohs Hardness 7.0 – 7.5

Specific Gravity 2.56 – 2.66

Refractive Index 1.542 – 1.578

Crystal System Orthorhombic

Luster Vitreous,Greasy

Cleavage Good

Fracture Uneven,Conchoidal

Color various

Birefringence 0.008

Dispersion 0.017

Pleochroism Strong trichroism: colorless to yellowish - pale blue - dark (violet)-blue

Mohs Hardness

Mohs Hardness: 7.0 – 7.5

1 Soft10 Hard

Crystal System

Value & Market

Price Tier Moderate ($50-500/ct)

Where to Buy Cordierite (Iolite)

Blue Nile Fine jewelry retailer

James Allen Gemstone jewelry

Brilliant Earth Ethically sourced gems

Amazon Wide selection

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About Cordierite (Iolite)

Cordierite, marketed as iolite, is a magnesium iron aluminum silicate with exceptionally strong trichroism—appearing deep violet-blue, pale yellow, and near-colorless from three different crystallographic axes—earning it the historical name 'water sapphire' and the sailors' compass stone, as Vikings may have used it to locate the sun on cloudy days. The finest gem iolite comes from Sri Lanka, India, and Tanzania, producing medium to dark violet-blue stones at very accessible prices. It remains an underappreciated alternative to blue sapphire.