Zoisite

Big Photo

Tanzanite
Tanzania
2.53 carats
© Palagems

The mineral zoisite is named after the collector Zois. It was first found in the Sau-Alp mountains in Austria, in 1805. It was originally called saualpite. Gem-stone quality specimens have only recently been found.

The gemstone members of the group are tanzanite, thulite and anyolite.

Zoisite Gemstones by Colour

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

Zoisite Gemstones by Size

This table shows distribution of Zoisite 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:1566.63 cts
Average:38.46 cts
Total photos:122
Do you have a larger Zoisite? Why not upload a photo?
0.20ct to 156.84ct156.84ct to 313.49ct313.49ct to 470.13ct470.13ct to 626.77ct626.77ct to 783.42ct783.42ct to 940.06ct940.06ct to 1,096.70ct1,096.70ct to 1,253.34ct1,253.34ct to 1,409.99ct1,409.99ct to 1,566.63ct
General Information
Varieties/Types:
Anyolite - Green Zoicite rock with black hornblende inclusions and large ruby crystals.
Tanzanite - A variety of Zoisite with a blue to blue-violet colour.
Thulite - A pink variety of Zoisite forming opaque masses.
Chemical Formula
Ca
 
2
Al
 
3
(SiO
 
4
)(Si
 
2
O
 
7
)(O,OH)
Ulrich Henn and Claudio C. Milisenda, Gemmological Tables (2004)
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Photos of natural/un-cut material from mindat.org
Zoisite Treatments
The mineral zoisite, Ca2Al3Si3O12OH, also occurs in a gem form called tanzanite, discovered only in 1967 (Hurlbut, 1969). Although a few, deep violet-blue dichroic crystals have been found, the majority of this material occurs with a brownish color having a strong violet-red/deep blue/yellow-green trichroism. Heating for about 2 h to 370°C converts the yellow-green component into a deep blue, leaving only a violet-red/deep blue dichroism and the desired color closely resembles sapphire. Some material is said to require a temperature over 600°C. The color is stable to 900°C, but above this temperature water is lost and a dirty yellow results. The causes of the color and color change have not yet been established with certainty; Gubelin and Weibel have summarized this and the gemology (Gubelin and Weibel, 1976) - Nassau (1984)
Heat-treatment of brownish-greenish-bluish crystals produces Tanzanite color - Blue Chart Gem Identification, Herve Nicolas Lazzarelli, 2010, p 3
Physical Properties of Zoisite
Mohs Hardness6 to 7
Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010)
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Specific Gravity3.20 to 3.50
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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FractureConchoidal
Arthur Thomas, Gemstones (2009)
Optical Properties of Zoisite
Refractive Index1.685 to 1.707
Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010)
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Optical CharacterBiaxial/+
Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010)
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Birefringence0.006 to 0.013
Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010)
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PleochroismStrong
Arthur Thomas, Gemstones (2009)
Dispersion0.0196
Arthur Thomas, Gemstones (2009)
ChatoyancyRare
Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010)
Colour
Colour (General)Green, brown, red-violet, bluish-green
Ulrich Henn and Claudio C. Milisenda, Gemmological Tables (2004)
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Causes of ColourBlue, (tanzanite - heat treated), V4+ in octahedral coordination; with V3+ in octahedral coordination. Brown-violet, V3+ in octahedral coordination (treatment turns blue, tanzanite). Green, Cr3+ in octahedral coordination. Pink (thulite), Mn3+ in presumably octahedral coordination
W. William Hanneman, Pragmatic Spectroscopy For Gemologists (2011)
TransparencyTransparent,Translucent,Opaque
Ulrich Henn and Claudio C. Milisenda, Gemmological Tables (2004)
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LustreVitreous
Arthur Thomas, Gemstones (2009)
Crystallography of Zoisite
Crystal SystemOrthorhombic
Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010)
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HabitPrismatic
Michael O’Donoghue, Gems, Sixth Edition (2006)
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Geological Environment
Where found:Occurs in medium-grade regionally metamorphosed crystalline schists formed from igneous, sedimentary or metamorphic rocks relatively high in calcium.
Michael O’Donoghue, Gems, Sixth Edition (2006)
Inclusions in Zoisite
"Fingerprint", graphite (black), hematite (hexagonal) - Blue Chart Gem Identification, Herve Nicolas Lazzarelli, 2010, p 3
Healing cracks, growth tubes, graphite - Gemmological Tables, Ulrich Henn and Claudio C. Milisenda, 2004, p 20
Further Information
Mineral information:Zoisite information at mindat.org
Significant Gem Localities
Pakistan
 
  • Gilgit-Baltistan
    • Shigar District
Blauwet (2006)
Tanzania
 
  • Arusha Region
    • Longido District
  • Manyara Region
    • Simanjiro District
      • Lelatema Mts
Dirlam et al. (1992)
[var: Tanzanite] Malisa (2003)
[var: Tanzanite] Malisa (2003)
[var: Tanzanite] Malisa (2003)
[var: Tanzanite] Malisa (2003)
[var: Tanzanite] Malisa (2003)
[var: Tanzanite] Malisa (2003)
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