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 Post subject: Globular brownish and blueish "stones" ?
PostPosted: Mon May 11, 2020 5:35 pm 
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I know of course that these are definitely no gemstones... Not even sure these are stones actually...A friend living in the Guadeloupe asked for some help in identifying strange pebbles he got in his garden. I am no mineralogist... Maybe someone among you have already seen such things ?


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 Post subject: Re: Globular brownish and blueish "stones" ?
PostPosted: Mon May 11, 2020 7:26 pm 
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Perhaps iron concretions.

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 Post subject: Re: Globular brownish and blueish "stones" ?
PostPosted: Mon May 11, 2020 9:13 pm 
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I'd suggest smashing one with a hammer and see what it looks like inside.


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 Post subject: Re: Globular brownish and blueish "stones" ?
PostPosted: Mon May 11, 2020 9:36 pm 
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Always a good idea (when dealing with nongems). If they're concretions tap lightly. They'll tend to have hollow areas and be crumblier than you'd expect

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 Post subject: Re: Globular brownish and blueish "stones" ?
PostPosted: Tue May 12, 2020 8:29 am 
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So, I asked the question and, as you nailed it, they are definitely crumbly. Some just under the finger and the biggest ones with the help of a tool.
Concretions, then. I don't know about the frequency nor the reason of these formations, but nothing very unusual I guess !


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 Post subject: Re: Globular brownish and blueish "stones" ?
PostPosted: Tue May 12, 2020 9:55 am 
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Probably the same mechanism as the formation of liesegang banding--precipitation of iron-rich minerals out of solution in pore space tends to organize into concentric bands because of local nucleation depleting the surrounding solution (this is probably also an important process in granites, per my undergraduate advisor's research).
Gemologically this process is responsible for some spectacular patterning in picture jaspers (particularly the greatest of all picture jaspers, Biggs from Oregon)
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 Post subject: Re: Globular brownish and blueish "stones" ?
PostPosted: Tue May 12, 2020 6:14 pm 
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Nice landscape !

Can we suppose that the 2 different colorations, blueish-brownish, as well in your picture as in mine, could be due to the state more or less oxydized of iron ?


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 Post subject: Re: Globular brownish and blueish "stones" ?
PostPosted: Tue May 12, 2020 6:58 pm 
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The blue in blue biggs jasper is from reduced iron, yes. On the concretions it could also be hydration of iron oxides to limonite kor some recrystallization of hematite, thin films can have a blueish look.

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 Post subject: Re: Globular brownish and blueish "stones" ?
PostPosted: Wed May 13, 2020 9:02 am 
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Thank you for all this, Stephen. :)


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