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I am trying to figure out if its a black star or gold sheen?
  
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JanGroen




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PostPosted: Jul 02, 2018 13:35    Post subject: I am trying to figure out if its a black star or gold sheen?  

Good day

My first mineral I am going to post is a 397 gram 1984 carat star sapphire. I am trying to figure out if its a black stare or gold sheen sapphire. It was donated to me with a cabinet filled with specimens.

Thank you
Jannie



IMG_0983.JPG
 Mineral: Sapphire
 Locality:
South Africa
 Dimensions: 70mm x 50mm x 50mm
 Description:
 Viewed:  833 Time(s)

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IMG_0975.JPG
 Mineral: Sapphire
 Description:
 Viewed:  826 Time(s)

IMG_0975.JPG



IMG_0981.JPG
 Mineral: Sapphire
 Description:
 Viewed:  827 Time(s)

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IMG_0997.JPG
 Mineral: Sapphire
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Duncan Miller




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PostPosted: Jul 03, 2018 00:22    Post subject: Re: I am trying to figure out if its a black star or gold sheen?  

JanGroen wrote:
I am trying to figure out if its a black stare or gold sheen sapphire. It was donated to me with a cabinet filled with specimens.

Hello Jannie - From a mineral point of view you have a corundum crystal with a polished end. Sapphire is gem corundum and gem names can be all over the place. As you live in the Western Cape, perhaps you would like to join the Cape Town Gem & Mineral Club ( http://ctminsoc.org.za/ ). The meetings are held in Bothasig and the members there will be able to help you identify your rocks, minerals and gemstones. Duncan

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JanGroen




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PostPosted: Jul 03, 2018 02:16    Post subject: Re: I am trying to figure out if its a black star or gold sheen?  

Duncan Miller wrote:
JanGroen wrote:
I am trying to figure out if its a black stare or gold sheen sapphire. It was donated to me with a cabinet filled with specimens.
Hello Jannie - From a mineral point of view you have a corundum crystal with a polished end. Sapphire is gem corundum and gem names can be all over the place. As you live in the Western Cape, perhaps you would like to join the Cape Town Gem & Mineral Club (ctminsoc dot org dot za). The meetings are held in Bothasig and the members there will be able to help you identify your rocks, minerals and gemstones. Duncan


Thank you I will look into joining the Cape Town club. This is a gem corundum know as a star sapphire. If cut correctly it will make a 6 rayed star on surface known as asterism. It is cause by the hemetite needles in the crystal structure. There is another new type of star sapphire in Africa, known as a golden sheen sapphire. I want to know if my stone is one of these gold sheen sapphires and how do one tell.
Jannie
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Duncan Miller




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PostPosted: Jul 04, 2018 01:36    Post subject: Re: I am trying to figure out if its a black star or gold sheen?  

JanGroen wrote:
...I want to know if my stone is one of these gold sheen sapphires and how do one tell.Jannie


Jannie, 'Gold sheen sapphire' is a marketing name for the corundum product from a particular Kenyan mine. There is a detailed description in The Journal of Gemmology (Volume 34(8), 2015, 678-691), which can be downloaded for free from the Gemmological Association of Great Britain's journal archive at gem-a.com. The 'sheen' effect seems to be due to reflection from (mostly) haematite platelets, unlike the asterism caused by rutile needles, although some of the 'gold sheen' material also shows asterism. There isn't any definitive gemmological test for 'gold sheen' corundum, other than appearance due to the inclusions. Unlike with mineral names, there is no standardisation of names for gem materials, and marketers can give novel gem varieties any names they like. At least 'gold sheen' is helpfully descriptive. Duncan

Here is the abstract of The Journal of Gemmology article:

"Sapphires displaying an attractive golden sheen reportedly were discovered
in eastern Kenya in late 2009, and were recently brought to market in
commercial quantities. The basic gemmological properties of the so-called
Gold Sheen sapphires are consistent with typical corundum. Microscopic
examination and Raman micro-spectroscopy showed that Fe-Ti oxides
(hematite, ilmenite and magnetite) are the main solid inclusions, and they
are oriented parallel to crystallographic directions in the basal pinacoid of
corundum characterized by a six-fold rotational symmetry. Hematite and
ilmenite are present in the form of exsolution intergrowths within platelets or
needles. Magnetite typically appears as larger and thicker black platelets. The
sheen effect in these sapphires originates from the simultaneous reflection
of light from the oriented network of exsolved hematite-ilmenite inclusions."

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