"Star of Kragerø" Cordierite

Photo Copyright © Jan Braly Kihle  - This image is copyrighted. Unauthorized reproduction prohibited.
Gemstone: Cordierite
Location of Origin:Kragerø, Telemark, Norway
Origin:Natural
Treatments:None reported
Weight:10.43 cts.
The "Star of Kragerø" 10.43 ct, discovered 1988, cut in 2001.

The cordierites from this locality are among the most Magnesium- and CO2-enriched species analyzed from the Bamble sector making its pleochroism white to pale bluish. Allotriomorphic cordierite crystals weighing up to 15 kg has been discovered close to this locality (e.g., image of the late Martin Frey of Univ. Basel holding one part of a 15 kg single crystal; Joseph Müllis is holding the other part, as seen in the daughter image below). Parts of this material are resorbed forming greenish patches consisiting of finegrained intergrowths of talc, kyanite, chlorite and quartz, less commonly also blackberry colored Magnesiodumortierite during counter-clockwise transition from Granulite facies into Whiteschist facies metamorphism. Nevertheless, larger gem quality domains are found as a rarity, their pale color turning them into beautiful facetted stones such as this one (cordierites from nearby Risør and Søndeled are richer in iron making larger gem cut material frequently look too dark).

Though the "Star of Kragerø" is of good gem quality, it still has a few inclusions of sillimanite needles and hexagonal flakes of hematite (normal to [001]) visible through a quality loupe. They are however all fluid inclusion free. This one of the largest cut Cordierites ever to come out of Norway. The rough uncut counterpart of this stone is exhibited at the Mineralogical Collection of the Mining Museum of Kongsberg.

Collection: Former collection of Jan B Kihle, now in the collection of the Mineralogical Geological Museum, University of Oslo.

More info on Bamble cordierites is available at (in Norwegian only): www.nags.net/Stein/1993/Cordieritt-Bamble.pdf

Kihle et al.,2010: Epitaxial quartz inclusions in corundum from a sapphirine–garnet boudin, Bamble Sector, SE Norway: SiO2–Al2O3 miscibility at high P–T dry granulite facies conditions. Journal of Metamorphic Geology, Volume 28, Issue 7, pages 769–784, September 2010


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