Sillimanite

saf-ahyuh r

Beautiful and rare, sillimanite is named for the famous American geologist Benjamin Silliman and was relatively unknown until a substantial find was discovered in Orissa, India in the 1990's. Sillimanite is not only scarce, but also difficult for miners to identify and problematic for cutters. These three attributes ensure that sillimanite remains a true exotic gemstone. Sillimanite ranges from colorless to white, brown, yellow, blue and green in color and consists of compact fibrous material that has a silky luster. A polymorph of kyanite and andalusite, sillimanite makes an exciting addition to any gemstone collection.

Sillimanite Polished
Sillimanite Classification
Common Name Sillimanite
Species Sillimanite
Sillimanite Optical Properties
Transparency Transparent - Translucent
Dispersion Strength: Weak Fire Value: 0.015
Refractive Index 1.659-1.680
Tolerance:(+0.004/-0.006)
Birefringence 0.015-0.021
Optic Character Biaxial
Optic Sign Positive
Polariscope Reaction Aggregate (AGG), Doubly Refractive (DR)
Fluorescence SWUV: inert or weak red
LWUV: inert or weak red
Pleochroism Trichroic, (in blue gems) strong blue, colorless, and light yellow; (in green gems) strong yellowish green, dark green, and blue
Sillimanite Characteristic Physical Properties
Hardness 6-7.5
Streak White
Specific Gravity 3.140-3.270 Typical:3.250
Toughness Poor
Inclusions Sillimanite is often vertically striated and they might contain rutile needles or hypersthene's fibers.
Luster Vitreous, Silky
Fracture Uneven
Cleavage Perfect, in one direction
Sillimanite Chemistry & Crystallography
Chemical Name aluminum silicate
Chemical Formula Al2SiO6
Crystal System Orthorhombic
Chemistry Classification Silicate

Sillimanite Colors

  • Black Sillimanite Black
  • Yellow Sillimanite Yellow
  • White Sillimanite White
  • Purple Sillimanite Purple
  • Colorless Sillimanite Colorless
  • Brown Sillimanite Brown
  • Blue Sillimanite Blue

Alternate Names

Fibrolite

Countries of Origin

Sri Lanka; Brazil; Madagascar; India

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Optical Phenomena

Cat's Eye

The term cat's eye, or chatoyancy, is used to describe a phenomenal optical property in gemstones, in this case, sillimanite. The effect, when present, appears as a bright, narrow slit similar to the pupils in the eyes of your favorite feline. This phenomenon is caused by parallel fibrous or needle-like inclusions that interfere with the passage of light throughout the crystal, scattering and reflecting light back to the viewer as a thin line. Best observed when gems are cut en cabochon, cat's eye sillimanite makes a phenomenal addition to any gemstone collection.

Cat's Eye Sillimanite
Cat's Eye Classification
Common Name Cat's Eye
Cat's Eye Optical Properties
Fluorescence SWUV: Blue:inert or weak red
LWUV: Blue:inert or weak red
Pleochroism Unobservable
Cat's Eye Characteristic Physical properties
Inclusions Sillimanite will appear fibrous is often vertically striated and they will contain parallel rutile needles or hypersthene's fibers.
Tim Matthews

Author

Tim Matthews

Tim Matthews is President and Chief Executive Officer of Jewelry Television® (JTV), as well as a member of the company's Board of Directors. He oversees and leads all aspects of the company's powerful omni-digital retail platform that uniquely specializes in fine jewelry and gemstones. His passion for business and gemstones has led him to become a recognized expert in the field of gemology. He is a life member of the Gemmological Association of Great Britain (Gem-A) and has earned Gem-A's highest degrees, the Gemmology (FGA) and Diamond (DGA) diplomas. He is also a Graduate Gemologist (GG) of the Gemological Institute of America (GIA) and has also completed GIA's Graduate Diplomas in Diamonds, Colored Stones and Pearls. Under his leadership, JTV has become the leader in the sourcing and selling of color gemstones and jewelry.

This page was created on June 27, 2014.

This page was last edited on October 24, 2019.