Sillimaite (named after professor Sillman of Yale University, USA) is blue to green, with distinct pleochroism showing pale yellowish green, dark green, and blue from different angles. When crystals occurs in long slender prisms in parallel groups, resembling fibres, the material is often called fibrolite.

Sillimanite is found in metamorphic rocks and occasionally in pegmatites. Blue and violet stones are found in Burma; greenish grey stones in Sri Lanka; fibrolite in Idaho (USA). Other sites include Czechoslovakia, India, Italy, Germany and Brazil.

Chemical Composition: 

Aluminium Silicate - Al₂SiO₅

Classification / Type: 

Polymorphous with Andalusite and Kyanite.

Colors / Varieties: 

• Transparent (all colors)
• Chatoyant (all colors)
• Star Stones (6-ray)

Crystal System / Forms: 

Orthorhombic System / Prismatic crystals with vertical striations on prism faces.

Hardness: 

7 - 7.5

Specific Gravity: 

3.23 - 3.28 (compact varieties: 3.14 - 3.18)

Cleavage / Fracture: 

Easy 1 directional cleavage parallel to the prism face / Uneven fracture.

Optic Character: 

Anisotropic, D.R.; Biaxial positive.

Lustre: 

Vitreous

Refractive Index / Birefringence: 

1.658 - 1.678 / 0.020

Pleochroism: 

Varies with depth of color.

Dispersion: 

0.015

Magnification: 

Very fine needles parallel to the cleavage direction, fingerprints and crystals.

U.V. Fluorescence: 

Mostly inert to U.V. rays. Colorless and paler shades fluoresce pink under both short and long wave.

Spectrum: 

Narrow bands at 462nm, 411nm and 410nm characteristic of darker colored stones.

Cause of Color: 

Iron

Specific Tests: 

Tendency to fracture due to easy cleavage.

Simulants (with separation tests): 

• Transparent Sillimanite: Labradorite Feldspar (R.I., S.G.), Beryl (optic figure, R.I., S.G.), Chrysoberyl (R.I., S.G.), Scapolite (optic figure, R.I., S.G., U.V. fluorescence), Spodumene (birefringence, U.V. fluorescence), etc.
• Sillimanite Cat's Eye: Moonstone (R.I., S.G.), Apatite (lustre, hardness, inclusions, spectrum), Chrysoberyl (R.I., S.G.), Quartz (R.I., S.G., cleavage), Glass (inclusions)

Geological Occurrence: 

Found in schists and gneiss, in metamorphic rocks.

Sources: 

Myanmar (Mogok tract), Sri Lanka, India (Orissa, Andhra Pradesh, Tamil Nadu), Kenya, Canada.

Cuts & Uses: 

Facetted cuts, cabochons, beads, etc.