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 Post subject: Australian saphires
PostPosted: Mon Sep 04, 2017 4:06 am 
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Looking around various online suppliers of sapphires I have noticed a tendancy
for the Aussy ones to be quite a bit darker than those from elsewhere.

Is this just a photography/browser/display thing or do the stones themselves tend to be darker
than elsewhere?

Is there any info anywhere on that type of question - ie. stones from place x tend to
be brighter - bigger - more included etc. I havn't stumbled on that type of data yet.
(I'm very new at this of course though)

john


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 Post subject: Re: Australian saphires
PostPosted: Tue Sep 05, 2017 12:08 am 
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Yes, this is a strong tendency in Australian stones, exacerbated by lighter stones often being sold mislabeled as from other locales. The best resource out there on sapphires is Ruby and Sapphire by Richard Hughes. He is as obsessive as he is expressive.

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 Post subject: Re: Australian sapphires
PostPosted: Tue Sep 05, 2017 6:58 am 
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Stephen Challener wrote:
Yes, this is a strong tendency in Australian stones,...


Thanks Stephen - It's hard to tell exactly what you're looking at on a screen sometimes.
I'll check out the book - thanks for the tip.

john


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 Post subject: Re: Australian saphires
PostPosted: Tue Sep 05, 2017 3:04 pm 
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A lot has to do with the geologic environment in which the sapphire forms.
Australian sapphires usually occur in igneous basalt. Lots of iron present often causing an inky look in the stones.

Sapphires found in metamorphic marble contain far less iron and are usually brighter.


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 Post subject: Re: Australian saphires
PostPosted: Tue Sep 05, 2017 3:49 pm 
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Barbra Voltaire wrote:
A lot has to do with the geologic environment in which the sapphire forms.
Australian sapphires usually occur in igneous basalt. Lots of iron present often causing an inky look in the stones.

Sapphires found in metamorphic marble contain far less iron and are usually brighter.


Interesting. Wikipedia says 5-14 FeO and 14% alumina in basalts so that makes sense.
I must admit I havn't given the chemistry a lot of thought as yet - I was just watching the
gold prospectors in australia on TV tonight though and they say the gold was pushed up with
other minerals by high pressure water from under the crust. They seem to have a really
active geology down there. (Or did have at one time)

thanks Barbra.

john.


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 Post subject: Re: Australian saphires
PostPosted: Tue Sep 05, 2017 4:26 pm 
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The basalt is likely just carrying up sapphires that formed at depth without having a direct chemical relationship (and I'd guess it's unlikely that iron would diffuse in significantly during transport?) but nevertheless basaltic sapphires do tend to be dark and iron-rich for whatever reason.

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 Post subject: Re: Australian saphires
PostPosted: Tue Sep 05, 2017 4:33 pm 
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I was always taught that the sapphires formed in the igneous basalt....just like they form in metamorphosed limestone.
I guess I stand corrected......possibly. :wink:


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 Post subject: Re: Australian saphires
PostPosted: Tue Sep 05, 2017 4:41 pm 
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Stephen Challener wrote:
The basalt is likely just carrying up sapphires that formed at depth without having a direct chemical relationship (and I'd guess it's unlikely that iron would diffuse in significantly during transport?) but nevertheless basaltic sapphires do tend to be dark and iron-rich for whatever reason.


I don't know enough geology, but if I had to guess - maybe it gets in as you say or perhaps there's
some in the mix at formation of the gem stage. Either way it's good to know I'm learning something
from all the trawling I'm doing !

john


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 Post subject: Re: Australian saphires
PostPosted: Wed Sep 06, 2017 12:09 am 
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Barbra Voltaire wrote:
I was always taught that the sapphires formed in the igneous basalt....just like they form in metamorphosed limestone.
I guess I stand corrected......possibly. :wink:

That has certainly been a hypothesis and I wouldn't say it's disproven or anything, but it does seem a bit difficult mineralogically. One telling sign is how etched they tend to be, indicating they weren't in chemical equilibrium with the host magma. Gems and Gemology had a great paper laying out the basalt-as-carrier model which I think is fairly convincing https://www.gia.edu/gems-gemology/winte ... e-levinson

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 Post subject: Re: Australian saphires
PostPosted: Wed Sep 06, 2017 9:43 am 
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Stephen Challener wrote:
The basalt is likely just carrying up sapphires that formed at depth without having a direct chemical relationship

Just the way it carries up diamonds, then.


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