nansaidh
spending too much on rocks
Jewelry artist, graphic designer, dog lover, car nut (etc.)
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Chert
Aug 22, 2008 14:11:05 GMT -5
Post by nansaidh on Aug 22, 2008 14:11:05 GMT -5
I can't seem to grasp this one. I've read the entire "rocks" part of a rocks & mineral book and although "chert" is mentioned a few times, it's never explained or defined. I assume this means its an aggregate term of some sort but can someone clarify it for me, please? Thanks! Nancy
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Sabre52
Cave Dweller
Me and my gal, Rosie
Member since August 2005
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Chert
Aug 22, 2008 23:36:38 GMT -5
Post by Sabre52 on Aug 22, 2008 23:36:38 GMT -5
For me at least, I try to remember that, though all are microcrystalline quartz, if it's associated with marine sedimentary rock like limestone and the silica is primarily biogenic ( from diatoms, radiolarian skeletons etc) it's usually flint or chert. If it's associated with igneous rocks, volcanic ash, rhyolite etc, it's usually jasper. As usual, the problem lies in common names which really confuse the issue. For example, Prairie Agate and Morgan Hill Poppy Jasper are chert, as the former forms up in limestone and the latter from radiolarian ooze ( at least in one theory). And then we have the issue of sedimentary agate like Dryheads and Fairburns which also come from limestone but are banded agates, and picture jasper which is really a silicified volcanic ash so is kind of sedimentary but of volcanic origin. And then there are tons of folks who use chert as the label for any plain looking jasper and lots of "Agate" that is more of a jasper/agate whereas "agate" used to stand only for banded varieties. I know I carry on but I'm totally into quartz family gems*LOL*.....Mel
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DeanW
has rocks in the head
Member since December 2007
Posts: 721
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Chert
Aug 23, 2008 0:37:03 GMT -5
Post by DeanW on Aug 23, 2008 0:37:03 GMT -5
And continuing with Mel's thoughts: when does an agate become chalcedony?
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Sabre52
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Me and my gal, Rosie
Member since August 2005
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Chert
Aug 23, 2008 6:52:31 GMT -5
Post by Sabre52 on Aug 23, 2008 6:52:31 GMT -5
Dean: Way I figure it is all agate is chalcedony but not all chalcedony is agate. Some definitions use "agate" to mean only banded chalcedony specimens while others say if the chalcedony is translucent, it's agate, opaque= jasper. Only problem with that is, flint is translucent chalcedony and chert is opaque microcrystalline quartz too. And then we have huge numbers of plume and moss agate types that are translucent chalcedony included with various opaque materials which would actually, I guess, make them jasper/agate. And so it goes on and on *L*....Mel
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nansaidh
spending too much on rocks
Jewelry artist, graphic designer, dog lover, car nut (etc.)
Member since August 2008
Posts: 311
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Chert
Aug 23, 2008 12:31:38 GMT -5
Post by nansaidh on Aug 23, 2008 12:31:38 GMT -5
Mel - I'm tickled to pieces with myself because I actually understood most of your explanation! LOL It was fascinating reading my new book explaining igneous, sedimentary and metamorphic rocks (along with the "how's and why's" which I usually crave). My biggest hurdle right now is translating what I see in a specimen to what I've read. It seems that in the end, one really needs to be at least part geologist to correctly identify a lot of rocks/minerals, at least by their confusing regional/common names (this issue is prevalent in conchology as well). I shall continue to read, learn, and ask countless questions. Thank you ! Nancy p.s. I've actually been on the path to this obsession since I was 8 or 9, when I found a gorgeous, softball-sized chunk of rose quartz at the edge of a plowed field at my uncle's farm on the eastern shore of Maryland. I still have that somewhere (packed away - sigh) - it's been a favorite paperweight of mine and I always wanted to get it polished.
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Chert
Sept 1, 2008 21:53:25 GMT -5
Post by cpdad on Sept 1, 2008 21:53:25 GMT -5
nans...want some more confusion ;D....maybe mel can help us out ;D....here is a sure piece of chert....it has a complete fortification band of agate/chalcodony around a vug full of crystals/minerals. i am totally lost on some explanations.....then a lot make sense.....then im lost again ;D....its still a cool piece...its savannah river chert.....kev,
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nansaidh
spending too much on rocks
Jewelry artist, graphic designer, dog lover, car nut (etc.)
Member since August 2008
Posts: 311
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Chert
Sept 2, 2008 11:46:47 GMT -5
Post by nansaidh on Sept 2, 2008 11:46:47 GMT -5
So the white pasty-looking stuff around the agate rings and inside the yellow is the chert?
Nancy
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nansaidh
spending too much on rocks
Jewelry artist, graphic designer, dog lover, car nut (etc.)
Member since August 2008
Posts: 311
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Chert
Sept 5, 2008 11:03:49 GMT -5
Post by nansaidh on Sept 5, 2008 11:03:49 GMT -5
Kev, They call it agatised chert. There is also opalised and moss chert. Confused yet? Ack! Completely!
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Sabre52
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Me and my gal, Rosie
Member since August 2005
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Chert
Sept 11, 2008 15:48:15 GMT -5
Post by Sabre52 on Sept 11, 2008 15:48:15 GMT -5
Kev, Your definitely right. I would call the banded area in that specimen a banded agate even though it's in a sedimentary chert/flint type material (ala Dryheads, Fairburn and Kentucky Agate) The new ranch we bought even has huge flint nodules that look like giant eggs and actually, when cracked open, have botryoidal chalcedony lined hollows which one would definitely call geodes. Just shows how much the microcrystalline quartz materials can resemble each other in every way except the genesis of the silica that creates the chalcedony......Mel
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Chert
Sept 11, 2008 20:18:52 GMT -5
Post by cpdad on Sept 11, 2008 20:18:52 GMT -5
nans....white pasty looking stuff is chert....whats the yellow stuff inside the band?...is up for debate ;D....it could be something opalised kinda looks like it....but sometimes its best to give up and enjoy the piece ;D.
mel....rockitman....i gonna go find a beer now ;D....yaw keeping me messed up again ;D....kev.
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