Sillimanite
India
6.13 carats
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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.
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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?
| General Information | ||||||||||||||||||||||
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| Varieties/Types: | Fibrolite - A fibrous form of Sillimanite. | |||||||||||||||||||||
| Chemical Formula |
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| Physical Properties of Sillimanite | ||||||||||||||||||||||
| Mohs Hardness | 6 to 7.5, Blue Chart Gem Identification (2010) More from other references | |||||||||||||||||||||
| Specific Gravity | 3.20 to 3.26, Blue Chart Gem Identification (2010) More from other references | |||||||||||||||||||||
| Cleavage Quality | Perfect, Gemmological Tables (2004) More from other references | |||||||||||||||||||||
| Optical Properties of Sillimanite | ||||||||||||||||||||||
| Refractive Index | 1.653 to 1.685, Blue Chart Gem Identification (2010) More from other references | |||||||||||||||||||||
| Optical Character | Biaxial/+, Blue Chart Gem Identification (2010) More from other references | |||||||||||||||||||||
| Birefringence | 0.014 to 0.021, Blue Chart Gem Identification (2010) More from other references | |||||||||||||||||||||
| Pleochroism | Strong trichroism is common for green stones: yellowish green - dark green - blue, Blue Chart Gem Identification (2010) More from other references | |||||||||||||||||||||
| Chatoyancy | Common grayish green to dark brown cat's eye due to grayish hypersthene's fibers (or rutile needles), Blue Chart Gem Identification (2010) | |||||||||||||||||||||
| Colour | ||||||||||||||||||||||
| Colour (General) | Colourless, blue-green, blue, green, gray-green, brownish, Gemmological Tables (2004) More from other references | |||||||||||||||||||||
| Causes of Colour | Blue, 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, Pragmatic Spectroscopy For Gemologists (2011) | |||||||||||||||||||||
| Transparency | Transparent,Translucent,Opaque, Gemmological Tables (2004) More from other references | |||||||||||||||||||||
| Fluorescence & other light emissions | ||||||||||||||||||||||
| Fluorescence (General) | May fluoresce reddish, Blue Chart Gem Identification (2010) More from other references | |||||||||||||||||||||
| Crystallography of Sillimanite | ||||||||||||||||||||||
| Crystal System | Orthorhombic, Blue Chart Gem Identification (2010) More from other references | |||||||||||||||||||||
| Habit | Prismatic, some material is fibrous, 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., 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 | ||||||||||||||||||||||
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