Joined: Mon Aug 28, 2017 10:39 am Posts: 62 Location: Birmingham - England
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)
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.
Joined: Sun Oct 16, 2005 12:22 pm Posts: 21600 Location: San Francisco
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.
Joined: Mon Aug 28, 2017 10:39 am Posts: 62 Location: Birmingham - England
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)
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.
Joined: Sun Oct 16, 2005 12:22 pm Posts: 21600 Location: San Francisco
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.
Joined: Mon Aug 28, 2017 10:39 am Posts: 62 Location: Birmingham - England
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 !
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.
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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