Gahnite

Big Photo

Bahia, Brazil
0.87 carats

Gahnite is named after J.G. Gahn, a Swedish chemist and mineralogist, who discovered the mineral. It was first described in 1807 for an occurrence in the Falu mine, Falun, Dalarna, Sweden.

Gahnite is a rare mineral belonging to the Spinel group. It has the same structure as Spinel but different chemistry (zinc).
7 photos

Gahnite Gemstones by Colour

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

Gahnite Gemstones by Size

This table shows distribution of Gahnite 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:0.20 cts
Heaviest:1.17 cts
Average:0.66 cts
Total photos:6
Do you have a larger Gahnite? Why not upload a photo?
0.20ct to 0.30ct0.30ct to 0.39ct0.39ct to 0.49ct0.49ct to 0.59ct0.59ct to 0.69ct0.69ct to 0.78ct0.78ct to 0.88ct0.88ct to 0.98ct0.98ct to 1.07ct1.07ct to 1.17ct
General Information
Chemical Formula
ZnAl
 
2
O
 
4
Michael O’Donoghue, Gems, Sixth Edition (2006)
Physical Properties of Gahnite
Mohs Hardness7.5 to 8
Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010)
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Specific Gravity4.00 to 4.62
Walter Schumann, Gemstones of the world (2001)
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Optical Properties of Gahnite
Refractive Index1.790 to 1.818
Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010)
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Optical CharacterIsotropic
Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010) Anomalous double refraction
Colour
Colour (General)Red-violet, different blue shades, green or blackish.
Walter Schumann, Gemstones of the world (2001)
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Causes of ColourBlue, Fe2+ in tetrahedral coordination
W. William Hanneman, Pragmatic Spectroscopy For Gemologists (2011)
TransparencyTransparent
Ulrich Henn and Claudio C. Milisenda, Gemmological Tables (2004)
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Crystallography of Gahnite
Crystal SystemIsometric
Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010)
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HabitOctahedra
Michael O’Donoghue, Gems, Sixth Edition (2006)
Geological Environment
Where found:Natural gahnite occurs as an accessory mineral in granites and granite pegmatites.
Michael O’Donoghue, Gems, Sixth Edition (2006)
Further Information
Mineral information:Gahnite information at mindat.org
Significant Gem Localities
Nigeria
 
  • Kaduna
Gems, Sixth Edition, Michael O’Donoghue, 2006, p. 413
Sri Lanka
 
  • Sabaragamuwa Province
    • Ratnapura District
      • Ratnapura
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