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Agate, Jasper and Chalcedony

Posted by Kristi Hugs  
Agate, Jasper and Chalcedony
August 02, 2009 08:29PM
I am trying to distinguish between Agate, Chalcedony and Jasper.

From what I am reading here at Mindat, Chalcedony is a cryptocrystalline variety of Quartz.
Agate is a banded Chalcedony, hence it is also a cryptocrystalline variety of Quartz.
Jasper is a microcrystalline variety of Quartz.
Bloodstone is a Chalcedony, which makes it cryptocrystalline, yet, it is also referred to as Blood Jasper. But Jasper is a microcrystalline variety of quartz, not a cryptocrystalline variety, right?

So is Blood Jasper just some name someone stuck on the piece because they decided it was not a Chalcedony? or are the inclusions different?

Any assistance would be most appreciated. Thank you in advance!
Re: Agate, Jasper and Chalcedony
August 03, 2009 05:54PM
    
Hello Kristi....sort of confusing, huh!! According to Bates & Jackson, "Dictionary of Geological Terms, 3rd Ed.",....."Cryptocrystalline, said of the texture of a rock consisting of crystals that are too small to be recognized and distinguished under the ORDINARY microscope" (emphasis mine).

And then, "Microcrystalline.......that are visible ONLY under the microscope". The difference between "ordinary microscope" and "microscope" is not stated.

That said, a distinguishing characteristic of chalcedony is, in thin section under the polarizing microscope, a "wavy extinction". All of the jaspars, agates, carnelions, etc., under the petrographic microscope have been cryptocrystalline and chalcedony.

For me, they are all simply varieties of quartz given different names both in antiquity and currently - "blood jaspar" seems maybe more accurate a name than "bloodstone", but bloodstone is the accepted name.

Just my opinion......

Don S.
Re: Agate, Jasper and Chalcedony
August 03, 2009 06:57PM
    
It's all quartz, sometimes with tiny inclusions of iron oxides etc.
Re: Agate, Jasper and Chalcedony
August 03, 2009 07:35PM
Thanks Don and Uwe :) See, the thing is, I am writing an article about the differences between Agate and Jasper. This will be to explain the differences to a very amateur crowd, So while I know they are all quartz, I am looking for distinguishable differences (microcrystalline and cryptocrystalline being one of those differences). I think what is throwing me off are things that are called Agate, but are actually NOT Agate specifically (like Moss Agate which has no concentric banding). So Moss Agate is actually more accurately identified as a Chalcedony.

Bloodstone is one that is difficult for me. It is not an Agate (no concentric banding) but also is not a Jasper....totally. I am assuming it is a Chalcedony with Red Jasper spots?

What seemed to be a really great idea in the beginning has ultimately confused the heck outta me......LOL

thanks again!!
Re: Agate, Jasper and Chalcedony
August 03, 2009 07:57PM
Chalcedony is translucent; jasper opaque. So "bloodstone" is a variety of jasper - green jasper with red spots.
Re: Agate, Jasper and Chalcedony
August 03, 2009 08:04PM
So am I correct in assuming that Jasper is NOT a type of chalcedony?

Thanks ya'll, this is really helping me. Just have to wrap my mind around this geological stuff >:D<
Re: Agate, Jasper and Chalcedony
August 03, 2009 08:30PM
Kristi,

Sorry for making things a bit more complicated for you but agate and chalcedony (also flint and chert) are not exactly straightforward varieties of quartz as they also contain a proportion of moganite (formula is SiO2 - so the same as quartz). Its a metastable species and in Britain it is confirmed in flint from Margate, Kent where its abundance is 20% (quartz being the other 80%). It is also thought to be present in these varieties of "quartz": beekite, carnelian, chrysoprase, hornstone, sard and sardonyx. I'm not sure if bloodstone contains moganite, but I think it might.

For further info - SiO2 is the chemical formula of other species too - tridymite, cristobalite, stishovite and coesite to name but four
Re: Agate, Jasper and Chalcedony
February 25, 2012 04:52PM
Hi Mira,

Did you ever write that laypersons' article about jasper, flint & quartz? I was just looking up some info on bloodstone and its relatives and came across this thread. I'd love to get a copy of your article if I may; even reprint it in our club bulletin if that is appropriate.

best,

Lizzie
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