Thomsonite

Thomsonite is named after Thomas Thompson, Regius Professor of Chemistry, University of Glasgow, Scotland, who first analyzed the mineral.

Thomsonite forms transparent to translucent brown, green, red, pink, grey, white, yellow, colourless thin, bladed prismatic crystals or fibrous masses.

Thomsonite forms radiating spherical or columnar aggregates which resemble Agate. This material is cut into cabochons.

The eye-like material usually comes from Isle Royale, Michigan, USA.
General Information
Chemical Formula
NaCa
 
2
Al
 
5
Si
 
5
O
 
20
· 6H
 
2
O
Michael O’Donoghue, Gems, Sixth Edition (2006)
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Photos of natural/un-cut material from mindat.org
Physical Properties of Thomsonite
Mohs Hardness5 to 5.5
Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010)
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Specific Gravity2.25 to 2.40
Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010)
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Cleavage QualityPerfect
Ulrich Henn and Claudio C. Milisenda, Gemmological Tables (2004)
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FractureUneven
Arthur Thomas, Gemstones (2009)
Optical Properties of Thomsonite
Refractive Index1.510 to 1.550
Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010)
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Optical CharacterBiaxial/+
Ulrich Henn and Claudio C. Milisenda, Gemmological Tables (2004)
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Birefringence0.015 to 0.020
Ulrich Henn and Claudio C. Milisenda, Gemmological Tables (2004)
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PleochroismNil
Arthur Thomas, Gemstones (2009)
DispersionDistinct-strong
Arthur Thomas, Gemstones (2009)
Colour
Colour (General)White, gray, yellowish, reddish. Partly with an agate-like banding
Ulrich Henn and Claudio C. Milisenda, Gemmological Tables (2004)
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TransparencyTranslucent,Opaque
Ulrich Henn and Claudio C. Milisenda, Gemmological Tables (2004)
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LustreSilky
Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010) Common silky lusterMore from other references
Fluorescence & other light emissions
Fluorescence (General)Patchy white common
Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010)
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Crystallography of Thomsonite
Crystal SystemOrthorhombic
Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010)
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HabitRadial fibrous structure with eye-like pattern (also banded)
Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010)
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Geological Environment
Where found:Thomsonite occurs in amygdules and fractures in mafic igneous rocks, most commonly basalts.
Michael O’Donoghue, Gems, Sixth Edition (2006)
Further Information
Mineral information:Thomsonite information at mindat.org
Significant Gem Localities
USA
 
  • Michigan
    • Keweenaw Co.
      • Lake Superior
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