Sillimanite

Big Photo

India
6.13 carats
© Rarestone.com

Sillimanite is named after Benjamin Silliman, Professor of Chemistry and Geology, Yale University, New Haven, Connecticut, USA.

Sillimanite forms transparent, rather slate-blue or blue-green slender poorly terminated prismatic crystals, and some material is fibrous.

Violet–blue stones have been reported from Mogok Stone Tract, Myanmar and greyish green chatoyant material is found in the Sri Lanka gem gravels.

Sillimanite is one of the most difficult gemstones to fashion.

Sillimanite Gemstones by Colour

This table shows the variety of hues this gemstone can be found in. Click on a photo for more information.
 
 
 
 
 
 

Sillimanite Gemstones by Size

This table shows distribution of Sillimanite gemstone sizes that are listed on this site. This can give a good indication as to the general availability of this gemstone in different sizes.
Contributed photos
Lightest:0.75 cts
Heaviest:63.15 cts
Average:7.50 cts
Total photos:37
Do you have a larger Sillimanite? Why not upload a photo?
0.75ct to 6.99ct6.99ct to 13.23ct13.23ct to 19.47ct19.47ct to 25.71ct25.71ct to 31.95ct31.95ct to 38.19ct38.19ct to 44.43ct44.43ct to 50.67ct50.67ct to 56.91ct56.91ct to 63.15ct
General Information
Varieties/Types:
Fibrolite - A fibrous form of Sillimanite.
Chemical Formula
Al
 
2
SiO
 
5
Michael O’Donoghue, Gems, Sixth Edition (2006)
Photos of natural/un-cut material from mindat.org
Physical Properties of Sillimanite
Mohs Hardness6 to 7.5
Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010)
More from other references
Specific Gravity3.20 to 3.26
Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010)
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Cleavage QualityPerfect
Ulrich Henn and Claudio C. Milisenda, Gemmological Tables (2004)
More from other references
Optical Properties of Sillimanite
Refractive Index1.653 to 1.685
Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010)
More from other references
Optical CharacterBiaxial/+
Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010)
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Birefringence0.014 to 0.021
Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010)
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PleochroismStrong trichroism is common for green stones: yellowish green - dark green - blue
Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010)
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ChatoyancyCommon grayish green to dark brown cat's eye due to grayish hypersthene's fibers (or rutile needles)
Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010)
Colour
Colour (General)Colourless, blue-green, blue, green, gray-green, brownish
Ulrich Henn and Claudio C. Milisenda, Gemmological Tables (2004)
More from other references
Causes of ColourBlue, Fe2+-O-Ti4+, charge transfer, probably similar to blue kyanite. Yellow, Fe3+ or Cr3+ in tetrahedral coordination. Brown, Fe features of yellow sillimanite plus inclusions of iron rich phase
W. William Hanneman, Pragmatic Spectroscopy For Gemologists (2011)
TransparencyTransparent,Translucent,Opaque
Ulrich Henn and Claudio C. Milisenda, Gemmological Tables (2004)
More from other references
Fluorescence & other light emissions
Fluorescence (General)May fluoresce reddish
Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010)
More from other references
Crystallography of Sillimanite
Crystal SystemOrthorhombic
Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010)
More from other references
HabitPrismatic, some material is fibrous
Michael O’Donoghue, Gems, Sixth Edition (2006)
Geological Environment
Where found:Sillimanite is found in high-grade metamorphic schists and gneisses, sometimes in pegmatites. Probably most gem sillimanite is found in gem gravels.
Michael O’Donoghue, Gems, Sixth Edition (2006)
Inclusions in Sillimanite
Hypersthene's fibers (or rutile needles)- Blue Chart Gem Identification, Herve Nicolas Lazzarelli, 2010, p 4

Oriented needle-like crystals - Gemmological Tables, Ulrich Henn and Claudio C. Milisenda, 2004, p 18
Further Information
Mineral information:Sillimanite information at mindat.org
Significant Gem Localities
Myanmar
 
  • Mandalay Region
    • Pyin-Oo-Lwin District
      • Mogok Township
        • Mogok Valley
[var: Fibrolite] Ted Themelis (2008) Gems & mines of Mogok
Ted Themelis (2008) Gems & mines of Mogok
        • Pein-Pyit (Painpyit; Pyan Pyit)
[var: Fibrolite] Ted Themelis (2008) Gems & mines of Mogok
[var: Fibrolite] Ted Themelis (2008) Gems & mines of Mogok
[var: Fibrolite] Ted Themelis (2008) Gems & mines of Mogok
Sri Lanka
 
  • Sabaragamuwa Province
    • Ratnapura District
No reference listed
Gems, Sixth Edition, Michael O’Donoghue, 2006, p. 451
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