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 Post subject: Natural vs. Synthetic Spinel (Chelsea Filter)
PostPosted: Tue Nov 23, 2010 1:21 am 
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Joined: Tue Nov 16, 2010 1:47 pm
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Hello,

I recently obtained a nice 1.5 ct, loupe clean, top blue stone in a parcel. The SG and RI positively mark it as either real or synthetic blue spinel. Its clarity and cobalt-color lead me to think it's probably synthetic.

When I use the chelsea filter on it, it showed a very dark red.

Is this different than natural spinel's chelsea filter reaction? What other determinative criteria should I be looking at to determine whether this is naural or lab created?

Thanks,

C.


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 Post subject: Re: Natural vs. Synthetic Spinel (Chelsea Filter)
PostPosted: Tue Nov 23, 2010 9:09 am 
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Joined: Tue Jan 29, 2008 8:55 am
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Location: Paris - France
Hi Chevalier,

Synthetic spinels often have a little bit higher RI than natural spinel so be sure your lecture was right. Only natural RED spinels can show a RI above 1.73. If you have a RI equal or superior to 1.73 and the stone is not red, it is most probably a synthetic.

Also natural blue spinels show under the spectroscope a ray in the blue (iron) at around 450 nm which synthetic material does not show.

The polariscope can also help you as often synthetic spinel shows a "cloudy" moving shadow when turned.

Under the Chelsea filter, both get red but synthetic spinel less than natural. On the other hand, fluorescence is often more important in synthetic than in natural spinels.

Hope this helps.

Best regards,
Christel

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 Post subject: Re: Natural vs. Synthetic Spinel (Chelsea Filter)
PostPosted: Tue Nov 23, 2010 2:31 pm 
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Joined: Sun Oct 16, 2005 12:22 pm
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Location: San Francisco
A Chelsea Filter will not separate natural from synthetic spinel.


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 Post subject: Re: Natural vs. Synthetic Spinel (Chelsea Filter)
PostPosted: Tue Nov 23, 2010 2:47 pm 
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Christel wrote:
Hi Chevalier,

Synthetic spinels often have a little bit higher RI than natural spinel so be sure your lecture was right.


Salut Christel,

Par "lecture" vous voulez dire "reading" en anglais?

Merci

C.


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 Post subject: Re: Natural vs. Synthetic Spinel (Chelsea Filter)
PostPosted: Wed Nov 24, 2010 9:48 am 
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Sorry Chevalier,

Yes, by lecture I meant reading. :wink:

Best regards,
Christel

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 Post subject: Re: Natural vs. Synthetic Spinel (Chelsea Filter)
PostPosted: Wed Nov 24, 2010 12:40 pm 
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Joined: Thu Jul 16, 2009 9:27 am
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Location: Sri Lanka
Quote:
If you have a RI equal or superior to 1.73 and the stone is not red, it is most probably a synthetic.


I have seen few Blue and dark blue Natural Spinels showing RI 1.730 and above (called Gahnospinel)

Best thing is spectroscopic test as Christel said. Blue Spinels that doesn't show strong 450 nm abs. line suspicious for synthetic. Fluorescence test conclusive for blue synthetic Spinels. UV long wave show bright red and SW glow will be bluish white. These only for Vernuiel synthetic Spinel

There is plenty of red Spinels (flux) in the market with RI same as natural. most stones shows natural like inclusions. only very very careful observation of spectrum helpful. Both shows chromium lines but natural shows 'organ pipe' lines


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 Post subject: Re: Natural vs. Synthetic Spinel (Chelsea Filter)
PostPosted: Wed Nov 24, 2010 3:04 pm 
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Quote:
. Fluorescence test conclusive for blue synthetic Spinels.


I'd be careful about this as there are natural cobalt blues which can also fluoresce. I'd say easiest conclusive test would be with a spectroscope. The cobalt blue synthetics have a very distinct spectrum which is very different from all natural blue stones


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 Post subject: Re: Natural vs. Synthetic Spinel (Chelsea Filter)
PostPosted: Wed Nov 24, 2010 6:40 pm 
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Joined: Mon Jan 15, 2007 6:48 pm
Posts: 835
Location: florida
Hi

Just from what i read , you got it in a parcel of gemstones , if so i could say synthetic ? but like Frank said spectroscope if you have one or take it to a gemologist to identify 8)


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