Sillimanite

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Physical & Optical Properties

Mohs Hardness 6,0 – 7,5
Specific Gravity 3,2 – 3,26
Refractive Index 1,653 – 1,685
Crystal System Orthorhombic
Cleavage Perfect
Color various
Birefringence 0,014
Pleochroism Strong trichroism is common for green stones: yellowish green - dark green - blue

Mohs Hardness

Mohs Hardness: 6,0 – 7,5
1 Soft10 Hard

Crystal System

a b c Orthorhombic a ≠ b ≠ c, α = β = γ = 90°

Value & Market

Price Tier Moderate ($50-500/ct)

About Sillimanite

Sillimanite is a fibrous to prismatic aluminum silicate polymorph found in high-grade metamorphic rocks, displaying colors from white and colorless to pale blue, green, or gray with a vitreous to silky luster. Transparent gemstone-quality sillimanite is rare and prized, with fine blue and pale green faceted stones from Myanmar and Sri Lanka treasured by collectors. It is the state mineral of Delaware and part of the andalusite-kyanite-sillimanite polymorphic trio representing different pressures and temperatures of metamorphism.

Jewelry Use

Durability 5/10

Science FYI Family

Frequently Asked Questions

What is Sillimanite?
Sillimanite is a fibrous to prismatic aluminum silicate polymorph found in high-grade metamorphic rocks, displaying colors from white and colorless to pale blue, green, or gray with a vitreous to silky luster. Transparent gemstone-quality sillimanite is rare and prized, with fine blue and pale green
What is the hardness of Sillimanite?
Sillimanite has a Mohs hardness of 6.0 to 7.5, making it moderate for use in jewelry.
What color is Sillimanite?
Sillimanite is primarily Various.
How much does Sillimanite cost?
Sillimanite falls into the Moderate ($50-500/ct) price range. Prices vary based on color, clarity, cut, and origin.
What is the crystal system of Sillimanite?
Sillimanite belongs to the Orthorhombic crystal system.

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