Iolite
Cordierite · Semi-Precious Gemstone / Mineral Specimen
Crystal System
Orthorhombic; typically occurs as short prismatic crystals, but commonly found as pebbles or massive aggregates in contact metamorphic rocks.
Mineral Group
Silicates; Cyclosilicate subgroup
Chemical Formula
Mg2Al3(AlSi5O18) to Fe2Al3(AlSi5O18)
Formation Process
Metamorphic formation in argillaceous rocks during regional or contact metamorphism. It also occurs in some igneous rocks like granites and pegmatites through magmatic crystallization.
About This Crystal
The specimen is a tumbled, water-worn, or polished pebble showing a deep violet-blue to greyish-blue color. It possesses a vitreous (glassy) luster and is translucent to nearly opaque. It exhibits visible internal cloudiness and a slightly mottled color distribution characteristic of 'water-sapphire' varieties.
Physical Characteristics
Color: Blue, violet, grey; Streak: White; Cleavage: Fair/Distinct on {010}; Fracture: Subconchoidal to uneven; Specific Gravity: 2.58–2.66; Fluorescence: Generally inert, but may show weak yellowish/reddish under SWUV.
Optical Properties
Refractive Index: 1.542–1.551; Birefringence: 0.008–0.012; Pleochroism: Strong (trichroic: violet, blue, and yellow-grey/brown); Optical Phenomema: Doubly refractive.
Hardness & Durability
Origin Region
Madagascar, Sri Lanka, India, Myanmar (Burma), and the Northwest Territories of Canada. Found primarily in magnesium-rich metamorphic rocks and some pegmatites.
Hardness & Durability
Mohs Hardness: 7 to 7.5; Toughness: Fair (brittle); Stability: Sensitive to heat and pressure due to cleavage planes. Good for jewelry but requires protective settings for rings.
Care & Maintenance
Clean with warm soapy water and a soft brush; avoid ultrasonic or steam cleaners. Store away from harder gems like sapphire or diamond to prevent scratching. Avoid sudden temperature changes.
Rarity & Value
Relatively common in smaller, included sizes; gem-quality transparent material is uncommon. Value is driven by color saturation and the absence of eye-visible inclusions. Treatments are extremely rare for Iolite.
Special Characteristics
Strong trichroism (color shift when viewed from different angles); occasionally exhibits chatoyancy (cat's eye) or aventurescence if hematite inclusions are present (Bloodshot Iolite).
Lore & History
Historically known as the 'Viking's Compass' (Stein-Viking) because thin slices were used as polarizing filters to locate the sun on overcast days for navigation. In metaphysical traditions, it is associated with the Third Eye chakra, believed to enhance intuition, vision, and creative expression.