Chrysoprase

 

last modified: Sunday, 19-Jul-2009 11:08:33 CEST

Document status: complete

Chrysoprase is a green chalcedony. It is quite rare and thus one of the more valuable quartz varieties. As lots of it comes from Australia, it is also known as Australian Jade.

 

Specific Properties

The color of chrysoprase is caused by inclusions of a specific nickel compound, the talc mineral willemseite. Compounds who contain bivalent nickel cations, Ni2+, are very often green. Thus, depending on the amount of the inclusion, the color can be more or less intense, but will normally only show small variations in its tone. Chrysoprase of good quality is evenly and deeply colored while still being translucent.

 

Occurrence

Chrysoprase develops at relatively low temperatures (in a range from 5 to 132C°, according to ->Skrzypek et al.) in weathered nickel-rich mafic rocks, in particular in metamorphically formed serpentinites in these rocks. Mafic rocks are dark volcanic rocks with a very low silica content. No quartz variety can be found in fresh, unweathered rocks of this type, all the silica in them is bound in silicates. Olivine, Mg2SiO4, often a major constituent in these rocks, tends to contain small amounts of nickel, Ni, as a replacement for Mg. When the olivine weathers to serpentine, the nickel is released and forms Ni-rich, typically green minerals, for example the aforementioned willemseite, (Ni,Mg)3[(OH)2 Si4O10]. These fine-grained minerals get trapped in the silica gel that is to become the chrysoprase.

 

Locations

Most chrysoprase comes from Australia, in particular from the Marlborough district in Queensland. Other sources of good material are Tanzania and Brazil. The only European and also the classical locations for chrysoprase are Szklary, and Wiry, both in Poland.

Poland


10mm  
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A  scan of a polished chrysoprase from Szklary. Note the homogeneous structure and the high translucency. At the bottom end you will see a thin layered structure, but this is an scanning artefact: To avoid Newton rings, I had to put the piece into a layer of water. Its meniscus causes distortions at the edges, but the flat parts are unaffected, of course.

Both locations in Poland have been mined for nickel, but the mines have been closed in the 1980s.


Tanzania


20mm  
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20mm  
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This  is a chrysoprase of good quality, very likely from a weathered nickel-rich serpentinite rock at the Hanety hill, north of Dodoma in central Tanzania. Its dull conchoidal fracture is typical for chalcedony in general. This surface structure makes the color look a bit patchy, but it is not, as can be seen in the second picture. It shows the same specimen, but from the other side, and illuminated from behind, to demonstrate its nice translucency and the very even and deep color.


 

Further Information, Literature, Links

The Chrysoprase Mines of Australia run a nice web page with information about and lots of pictures of chrysoprase.

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