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Bev

Big Stromopoid Agate?

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Bev

Given what Precambrian Man showed for stroms, here is another odd rock...

attachicon.gifodd rock top.jpg

Could that one possibly be a stomolite?

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Tethys

Bev

Could that one possibly be a stomolite?

It could be a stromatolite, or it could be algal matgrounds with alternating layers of sand.

I found this excellent site on stromatolites and stromatoporoids. Many excellent photographs of both and good information on their various forms of preservation.

There are even a few stromatoporoid slices in a lovely pale honey colored banded chert from Ontario.

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Wrangellian

That's a good site, Teth... You've got your reading but out for you now, Bev!

I am not sure what you're saying, Ed, I have seen lots of milky agates/chalcedony incl. some of those Oregon ones that I have some examples of, though lacking banding, but the banding in that one looks sharp enough to be banded agate, to my eyes. There is lots of variation in agate from milky to clear to various banding and inclusions, and assuming this is Mohs 7 and not 3 then it is either chert or agate/chalcedony, and as Howard pointed out, chert is rarely so translucent (albeit milky) as this one appears to be. I'll admit I could still be wrong about whether it is agate over chert but I see nothing inconsistent with agate (having only the pics to go by!). The other chunks of quartz that Bev has shown I agree are consistent with the regular massive-crystalline quartz that we find everywhere, but chalcedony in my experience breaks somewhat more like glass because it is more amorphous. Bev's piece appears to have broken like this and then been weathered/eroded.

Where is PAINSHILL?? He's letting us flail around again...

BTW Gotland is (sort of) part of Sweden.

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Mr_ed

I must have misunderstood Bev.. I thought she said it was softer than agate.

I agree that you can call it agate if it is hard enough... but I have thrown pieces away that were of similar color and clarity. I would not call it agate but if it is hard enough it qualifies as agate I agree and I said I wouldn't call anyone wrong for calling it agate.. but since chalcedony, in my mind is called agate when it meets certain visual appeal and agate is agate to set it apart from quartz because of its visual appeal.. I would not call that rock agate.. I would call it quartz or chalcedony (if it is hard enough) but it doesn't have the visual appeal for me to call it agate.. close I admit.. but not quite..

I guess I am really just confusing the issue... so lets call it agate if it has a hardness of 6.5 to 7. and move along.

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Mr_ed

Eric

See Bev's post # 31 ... She says it scratches with glass..

Cheers

Ed

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Bev

Eric said that Quartz scratches Quartz. I used a quartz crystal to scratch it.

It DOES NOT scratch with a kitchen knife.

Agate?

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Mr_ed

Yes that would mean it is agate, or of the same family of chalcedony.

Cheers

Ed

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Wrangellian

Ah, I guess it's just a terminology issue - As someone stated, if we're not dealing in exact scientific terms, just colloquial, confusion sets in! One person's Agate is another's Leaverite. I am using 'agate' and 'chalcedony' interchangeably (easier to type 'agate'!)... Chalcedony sure, but given the banding I thought Agate was appropriate.

(It's like snail vs gastropod - maybe by some definitions they are not the same but I just call them all snails to save on typing... then I ramble on in other areas anyway!)

Edited by Wrangellian

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Mr_ed

I haven't dealt with semi precious stones for about 20 years so I have not learned as much as I could have and I have forgotten some of what I did know, but I learned some things and had my memory refreshed about some things on this thread. It is good to debate this stuff.. wouldn't learn anything otherwise..

Thanks

Ed

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Tethys

Wrangellian ~

I guess it's just a terminology issue - As someone stated, if we're not dealing in exact scientific terms, just colloquial, confusion sets in!

Yes, all of the variety names for silica dioxide are confusing. In Mn we have Gunflint Chert, which is black, semi-translucent chert that contains microfossils, and makes excellent flints due to its iron content. We have BIF which is chert, and it's associated stromatolites which are known as Mary Ellen Jasper

The only official agate is the Lake Superior variety which is commonly carnelian* red with white banding. It is appx 1 billion years old, formed as amygdules in the Keweewanean basalt flows, and the silicates can either be primary, or secondary replacements of other minerals.

There is an entire online book entitled The Other Lake Superior Agates that is a pretty comprehensive collection of all of the various forms in addition to the basic banded agate. The link is a large Pdf, and the photos will make any agate lover drool.

Silicified dolomite ie. chert is common in all of our paleozoic strata, and it can often display agatized areas but it has a lower specific gravity and hardness than LSA. Thus agate is generally used to refer only to LSA formed in volcanic rocks, and the silicates that formed via replacement of carbonates in the various sedimentary rocks are cherts, jaspers and siltstones. If they display banding they are banded cherts.

Near Bev there is even a archaeology site named Grand Meadow where chert was quarried for stone tools. The photo at the link shows the range of earth tones typical of the mid to late Ordovican cherts.

*(ack..yet another term for S1O2!)

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Wrangellian

I haven't dealt with semi precious stones for about 20 years so I have not learned as much as I could have and I have forgotten some of what I did know, but I learned some things and had my memory refreshed about some things on this thread. It is good to debate this stuff.. wouldn't learn anything otherwise..

Thanks

Ed

Me too!

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Wrangellian

Silicified dolomite ie. chert is common in all of our paleozoic strata, and it can often display agatized areas but it has a lower specific gravity and hardness than LSA. Thus agate is generally used to refer only to LSA formed in volcanic rocks, and the silicates that formed via replacement of carbonates in the various sedimentary rocks are cherts, jaspers and siltstones. If they display banding they are banded cherts.

So you are suggesting that Bev's piece is chert, then?

I only suggested LSA because I wasn't sure whether Bev's collecting area was LSA area. That doesn't mean other agates might not be found in her area though, I know agates can be found in a lot of places. But if it's chert, that point is moot!

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Bev

Two things...

A. We have a lot of glacial till down here that comes from up North. Lots of hard head granite stones were left by the glaciers.

B. Give that the rock was here before me, we do not even know if it is a Minnesota rock or from someplace else. Like, I've brought Montana rocks back here and they are in the gardens too.

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Coco

*(ack..yet another term for S1O2!)

Si O2 !;)

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Precambrian Man

Bev.., after a little research and some reading [ something i should have done before i posted ] i now believe i am 100% wrong. I will tend to agree with others that indicate this material as being a chert. After viewing photos and reading that chert can be opaque to translucent and exhibit banding characteristics, it resembles chert more than it does agate

I should have only stated that it looks to be from the class of silicates , Squalicorax said it well! is it a strom?, quite possibly!

Should stick to what i know of from my local area. anyhow, got to go! dinners up! main course: crow , desert: humble pie! thanks. :)

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Bev

Banded Chert it is! But it is a fossil and that makes my day! :D

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Mr_ed

I don't think anyone has said that it is a fossil Bev.. but don't let me spoil your day..

Cheers

Ed

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Bev

Hard to say if this is a silicified stromatoporoid or banded chert.

Due to the curvature and mound like shape i'd say this is a stromatoporoid.

Squalicorax did in the first reply. :D

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Mr_ed

Well he may be right .. I am sure he is more knowledgeable than I am about such things..

I wonder if he could provide a example of a similar one. for the doubtful ones ..

I don't believe it looks fine enough grained to be chert...

Did you look at it with a magnifying glass as suggested in that article on stromatoporoid vs stromatolite

I think it would be impossible to confirm that it is such by looking at it given that it looks more like LSA or quartz than anything I can find to compare.

I collect artifacts and lots of them are chert.. It has a particular sheen and fine grained look to it.. that that rock does not have so I have trouble with someone confirming that to be a without having it in his hand, sorry ..

Ed

Edited by Mr_ed

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Bev

So, the science teacher who has taught in this area for over 35 years, Gary Erickson, stopped by. I wasn't home or in back in the garden. Here is what Gary emailed me today about the rock...

"...the one it is an agate type rock formation, and not a chert. The line between chert and agate is at some point a blurred line as both are liquid formed forms of quartz related rocks, but that one definitiely is in the agate group."

So, honkin' big agate plus strom...

:D:meg dance:

I SINCERELY want to thank EVERYONE who has partaken of this conversation. What can be obvious in hand is often a challenge with photos.

THANK YOU EVERYONE!!!

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Mr_ed

Well I collected agates for a long while when I was younger .. I have learned more from this thread about agate and how it is formed and how it is associated to chert and jasper and chalcedony and about stromatolites and such than I ever did when I was collecting.. so thank you for posting ..

Eheers

Ed

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Wrangellian

Agate/chalcedony is very varied.. Agates in one area can look completely different from agates in another area.

It certainly does help to have something in hand to ID it, if not seeing a number of other similar specimens rather than just the one, or knowing stuff about the local geology/its point of origin.

Edited by Wrangellian

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Precambrian Man

Agate/chalcedony is very varied.. Agates in one area can look completely different from agates in another area.

It certainly does help to have something in hand to ID it, if not seeing a number of other similar specimens rather than just the one, or knowing stuff about the local geology/its point of origin.

I guess i was correct in my initial assesment/guess on type of material. Any flip flopping allowed on this FF?

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Wrangellian

Flip-flopping better be allowed!

I think there are still varied opinions on this.. should we take a vote?

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Bev

Lordy, about the only way we can do a vote is to pass the piece around via snail mail so that everyone can see it in hand - and it would disappear somewhere...

Precambrian man, I think you were right on. The only thing we really know is that it is not chert as the science teacher sees it - everything else is still up for debate by someone. :)

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