navi
starting to spend too much on rocks
Member since November 2008
Posts: 229
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Post by navi on Nov 21, 2008 20:50:54 GMT -5
ok, i'm confused. i have been a rockhound for years and feel like an idiot for not knowing this. until recently, i've always been into mineral specimens. now, i want to tumble stones and enhance nature's beauty.
just what am i tumbling? that is what i want to know.
chalcedony seems to be a name that covers a LOT!!! chert and flint fall in line with it. what is the difference? is flint a higher grade chert, or is it the same, just being called something else in a different region. agate seems to be a chalcedony that isn't one solid color everywhere. it could be banded, etc. what exactly is jasper? is that solid color chalcedony, or maybe a mix? sorry, if this has been covered 1.000,000 times already.
where i'm at, there is banded agate, which is easy enough. there is an abundance of chert, (or is it flint?). i just found an area that has chert that is white or blue, with zebra stripes. also, solid olive green, pink, red, blue, grey, purple. i've found one rock that had all of those colors. most are combos. i think that one is mozarkite, of course, a fancy name for chert.
which brings me back to the original question, can somebody (in idiot terminology, since i am one) define all the general names of chalcedony? not the specific sub types, just the differences between agate, jaspar, flint and chert.
good luck! ;D thank you for your help.
navi
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Sabre52
Cave Dweller
Me and my gal, Rosie
Member since August 2005
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Post by Sabre52 on Nov 22, 2008 20:03:08 GMT -5
OK, I'll give it a go *S* Chalcedony, flint, chert, agate etc are all microcrystalline quartz. Jasper is kind of more fibrous massive microcrystalline quartz. Now, here comes the confusion we get into with common names. Chert is a term sometimes used to describe poor quality jasper but in actuality, chert and flint come from marine sedimentary deposits whereas jasper comes from volcanics or metamorphosed volcanic sediments. True jasper is supposed to be opaque but then we get into the common named jasper like Stone Canyon etc which are part agate which we should call jasper/agate. Flint and chert can be opaque or translucent either entirely or in part. Agate used to be a term which applied only to true banded agate but now is used as common names for lots of included jasper/agates ie. Moss agate, plume agate etc. Chalcedony is usually thought of as clear to colored microcrystalline quartz with the norm being basically clear colorless to gray, red being carnelian, yellow being sard etc. Technically, agate should come from deposits associated with volcanics ie, Brazilians, Botswanas, Lakers etc. However, when we get into common names, Tee Pee Canyons, fairburns, Dryheads, Kentucky's etc are all called agate because of the banding when they are sedimentary in origin and should probably be classed as banded chert. So, I guess to summarize, and some may not agree:
All are microcrystalline or microfibrous quartz ( chalcedony) with various amounts of mineral inclusions which give the wide variety of color and pattern.
Agate ( common name) is banded and can be sedimentary or volcanic in origin.
Jasper and agate are basically volcanic in origin. Volcanic agate and jasper are often associated with ash, rhyolite and basalt formations.
Genesis of the silica in volcanic agate/jasper is overlaying ash deposits, Genesis of the silica in cherts and flints is thought to be organic sources like sponge spicules and radiolarian skeletons.
Flint and chert are sedimentary in origin ( often associated with limestone deposits) Oh yeah, and Some definitions classify flint as a fine quality translucent subdivision of chert with chert being more opaque but some chert is quite translucent and to my way of thinking much like flint.
Agate + jasper...volcanic
I'm sure I've messed up this explanation somewhere and lots of folks might disagree with my thinking.*L* as the common lapidary material names mess me up all the time. For instance, lots of jaspers are not jasper at all. Poppy jasper is more of a chert and lots of picture jasper is kind of a silicified volcanic mud much like rhyolite in texture and makeup.
Hopes this helps a bit anyway. If you have additional questions. I'll try to address them *LOL* but no promises I'll make sense...Mel
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navi
starting to spend too much on rocks
Member since November 2008
Posts: 229
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Post by navi on Nov 22, 2008 23:05:41 GMT -5
thanks, mel. that did help some. i live in eastern missouri, by st. louis. i believe we're more sedimentary/limestone around here, so, i guess jaspar is out, for local finds.
or, am i wrong.......................?
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Sabre52
Cave Dweller
Me and my gal, Rosie
Member since August 2005
Posts: 20,466
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Post by Sabre52 on Nov 23, 2008 10:04:44 GMT -5
There could be jasper and agate present from river transported or glacial deposits from the Great Lakes region ( Daviess, Gentry, Grundy, Livingston county etc. ) but I think most of what you'd find would be flint and chert. associated with limestone deposits. Flint and chert can be pretty nice stuff though ( Mozarkite for instance in Benton and Hickory counties and along the Current River in Ripley county. SE Missouri has the Missouri Lace Agate in the old lead mines area. Crowley Ridge in southern Missouri is famous for neat banded agate....Mel
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Post by sparkles on Nov 23, 2008 13:23:28 GMT -5
Hiya Folks,
I have little to add other than a further subdivision on the Flint / Chert. I live in Norfolk UK - and we have a lot of Flint here - go a little west and it's Chert, everything Mel has described is exactly how I understand it - and to add that Flint here is only found in Chalk, and the Cherts are found in Limestones.
Congratulations Mel! You did the best job I've ever seen of clarifying this point, it's something I'm always grappling with one way or another! ;D Thanks Sir!
Cheers,
Sparkles.
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Sabre52
Cave Dweller
Me and my gal, Rosie
Member since August 2005
Posts: 20,466
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Post by Sabre52 on Nov 23, 2008 14:02:44 GMT -5
Sparkles: I've always wondered about that. Somewhere, I've got a nodule of Nevada flint that came from chalk and it looks almost identical to English flint. The flint from chalk seems much more pure and fine textured than for instance, our Texas flint which comes from limestone. Texas flint can be nice and translucent but often has a chalky exterior where it grades from limestone to flint. Interesting topic anyway...Mel
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Post by Hard Rock Cafe on Nov 23, 2008 14:56:41 GMT -5
Good explanation, Mel! The common names really add to the confusion.
I've been tumbling some west Texas material that has a white (but not chalky) exterior, but a colored interior. I thought it was just dried out or had a desert varnish. Now I'll have to see whether I should actually call it flint.
Chuck
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Post by sparkles on Nov 23, 2008 15:05:06 GMT -5
Bingo Chuck! You hit it on the head there Buddy - that crusty skin on flint nodules is due to the loss of water from it's matrix leading it to degrade. It takes forever to grind that off the tumbles! He he he! I take an easy approach to what's a flint and what's not, as with all things the border is often very blurry! he he he! Mel :- Chalk flint often has a much more pure appearance although not always - I tend to find it in Black and Brown translucent versions. Often the only way to distinguish one from another is to chip it with a hammer, flint has a much nicer fracture and doesn't tend to leave as many angular chips as the cherts around here. I agree - Fascinating subject... Thanks Navi! As you can tell - it's not as clear cut as one might think it should be ;D he he he! Cheers, Sparkles.
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navi
starting to spend too much on rocks
Member since November 2008
Posts: 229
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Post by navi on Nov 23, 2008 23:10:32 GMT -5
thanks all. this helps. chert it is. but, is chert chalcedony? ?
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Sabre52
Cave Dweller
Me and my gal, Rosie
Member since August 2005
Posts: 20,466
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Post by Sabre52 on Nov 23, 2008 23:18:19 GMT -5
Navi: Short answer: Chert is a type of primarily opaque included chalcedony. The colors and patterns, lack of transparency etc are caused by inclusions of other minerals into the clear chalcedony. The less dense the inclusions, the more transparent or translucent the chert...Mel
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