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Post by MsAli on Oct 29, 2018 9:32:12 GMT -5
I've always called this chert, but what makes it not a jasper/agate? I know technically agate and jasper is a form of chert Who decides what classification it is? It has a a bunch of little druzy pockets, banding, is opaque Pretty purple, pink and grey
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Deleted
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Post by Deleted on Oct 29, 2018 11:10:03 GMT -5
Because I see banding, I'd call it agate. Agate has banding (and/or, if translucent, moss, plume and other agate characteristics). Flint is chert that has formed in limestone deposits. Jasper is chert that formed from volcanic deposition. The term "chert" alone refers simply to opaque chalcedony (usually pieces that exhibit little color or pattern).
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Post by MsAli on Oct 29, 2018 11:29:38 GMT -5
Because I see banding, I'd call it agate. Agate has banding (and/or, if translucent, moss, plume and other agate characteristics). Flint is chert that has formed in limestone deposits. Jasper is chert that formed from volcanic deposition. The term "chert" alone refers simply to opaque chalcedony (usually pieces that exhibit little color or pattern). Thank you Could the outside matrix of this be limestone?
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Post by Deleted on Oct 29, 2018 11:37:01 GMT -5
Could the outside matrix of this be limestone? Might be. I've also seen similar rind on petrified wood and agates that formed in both caliche and rhyolite. You could test for limestone with a drop of muriatic acid (depending on hardness, you may get an instant bubble reaction or have to wait a bit and observe closely). Some caliche will fizz when exposed to acid, so not conclusive. Caliche is much softer than limestone, however.
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Post by MsAli on Oct 29, 2018 11:39:35 GMT -5
Could the outside matrix of this be limestone? Might be. I've also seen similar rind on petrified wood and agates that formed in both caliche and rhyolite. You could test for limestone with a drop of muriatic acid (depending on hardness, you may get an instant bubble reaction or have to wait a bit and observe closely). Some caliche will fizz when exposed to acid, so not conclusive. Caliche is much softer than limestone, however. Thank you
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Post by fernwood on Oct 29, 2018 12:38:40 GMT -5
Agate in my book, due to the irregular banding. Agate often has the druzy pockets.
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Post by MsAli on Oct 29, 2018 12:47:45 GMT -5
Agate in my book, due to the irregular banding. Agate often has the druzy pockets. When you asked me the other day why did I call it Chert, I got to thinking. I guess we have always considered "agate" in Mn to only be Lake Superior Agates Even Google searches on Agates in MN will only show you LSA But you would think a region so geological rich must have other types of agates right? To me it is really beautiful. I am sending this chunk to be knapped, but I have 4 others that I will slab and see how it cabs.
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Post by fernwood on Oct 29, 2018 13:00:15 GMT -5
Great. Same thing in WI. I was fortunate as a child to have some adult family friends who were into lapidary. By the time I was 10 years old, I was taught the differences. There was lots of chert in NW WI where I grew up. Agates there were elusive on my families land. When I went to college, an hour south, agates were plentiful. Still have some of those finds, 35 years later.
Talk to Billy about his agate finds in MN. Much more than the LS variety.
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Post by rockjunquie on Oct 29, 2018 13:04:02 GMT -5
That was a really great question!
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Post by stephan on Oct 29, 2018 14:15:30 GMT -5
I'd say agate, as well. Mostly cloud agate, but I see fortifications as well. As for what makes something "chert," here's the best definition I've heard: "If you like it, it's jasper, if you don't, it's chert." 🤣
Please do post the knapping/cabbing results.
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NRG
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Post by NRG on Oct 29, 2018 14:36:10 GMT -5
Because I see banding, I'd call it agate. Agate has banding (and/or, if translucent, moss, plume and other agate characteristics). Flint is chert that has formed in limestone deposits. Jasper is chert that formed from volcanic deposition. The term "chert" alone refers simply to opaque chalcedony (usually pieces that exhibit little color or pattern). Thank you. I have seen some define chert as formed in freshwater limestone and flint in ocean born limestone. I may have this backwards. Sabre52 may chime in here. From my studies Agate is the purest of these things. Jasper's formed in volcanic mud. Usually iron bearing for the various colors. But all four things Agate, Jasper, Chert, Flint all are essentially the same thing, but formed in different settings. Not disagreeing with you. Hoping to add to the discussion.
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Post by MsAli on Oct 29, 2018 14:55:08 GMT -5
mn.gov/admin/assets/stone-tools-of-minnesota-part2_tcm36-247479.pdf"The first and most important group is the cryptocrystalline silicates, which consist principally of microscopic silica crystals and only minor traces of other elements or minerals. These materials usually occur in sedimentary formations, and are thought to result from the chemical precipitation of silica from seawater, the gradual replacement of other minerals by silica, or other processes (see Calvert 1983; Hesse 1989). If the crystals are arranged in a fibrous structure, the material is called chalcedony. If the crystals have random orientations, somewhat like a felt of silica crystals, the material is called chert. These two materials are easily distinguished by petrographic analysis, but not by macroscopic visual inspection. Agate is a variety of chalcedony usually having concentric bands of different colors, although other patterns (such as moss agate) do occur. Jasper is a variety of chert that is relatively opaque and usually yellow, olive, brown, or red in color as a result of the presence of iron. Flint may be used as a synonym for chert, or may refer to varieties of chert that are opaque to translucent and dark grey, dark brown, or black in color. The dark color may come from the inclusion of organic matter. In practice, archaeologists often use flint, chert, jasper, and chalcedony as mutually exclusive, descriptive terms rather than according to their more technical definitions. In this type of usage, chalcedony is a high-quality, translucent to transparent silicate, often with a waxy texture. Flint is highquality, translucent to opaque rock that is dark in color. Jasper is good-quality, usually opaque, and sometimes waxy-textured rock that is yellow, brown, red, or olive in color. And chert is more opaque, often lighter colored or somewhat coarser cryptocrystalline material that does not look like chalcedony, flint, or jasper. This descriptive usage is acceptable, especially when a petrographic identification is not available"
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Post by orrum on Oct 29, 2018 14:59:53 GMT -5
Old timer told me chert is plain looking and jasper is purty!!! Works for me.
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Post by MsAli on Oct 29, 2018 15:20:13 GMT -5
Swan River Chert (SRC) (Campling 1980; Bakken 1985, 1993) is present in glacial drift in the Southern Resource Region. Although it is found at many archaeological sites in the region, it is probably not as important here as in the Western Resource Region. As discussed above, SRC has a vast distribution to the north and probably also occurs into parts of northern Iowa. Also as discussed above, this material is extremely variable, especially in color. The “orange peel” texture of fracture surfaces, cloudy yellow color of transmitted light, and microscopic, agate-like banding are normally the most useful diagnostic characteristics. The spongy or ropy texture of the cortex is also distinctive, when cortex is present. SRC might be mistaken for Warsaw Chalcedonic Chert (from Iowa) (Morrow 1984, 1994) or certain varieties of Prairie du Chien Chert, although careful examination should serve to distinguish SRC. Neither of these materials seem to exhibit the microscopic, agate-like banding seen in SRC or its distinctive cortex.
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Post by MsAli on Oct 29, 2018 15:24:09 GMT -5
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Post by fernwood on Oct 29, 2018 16:24:31 GMT -5
All about regional terms.
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NRG
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Post by NRG on Oct 29, 2018 16:52:47 GMT -5
Could the outside matrix of this be limestone? Might be. I've also seen similar rind on petrified wood and agates that formed in both caliche and rhyolite. You could test for limestone with a drop of muriatic acid (depending on hardness, you may get an instant bubble reaction or have to wait a bit and observe closely). Some caliche will fizz when exposed to acid, so not conclusive. Caliche is much softer than limestone, however. Caliche is a cement composed mainly of calcium carbonate. en.m.wikipedia.org/wiki/Caliche
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gemfeller
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Post by gemfeller on Oct 29, 2018 17:06:37 GMT -5
Quartz classification is messy and new research adds to the ongoing confusion. Here's a site I've found enlightening in many ways and I post it here for those who are not aware of it. Enjoy! www.quartzpage.de/#
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Post by MsAli on Oct 29, 2018 17:22:01 GMT -5
Quartz classification is messy and new research adds to the ongoing confusion. Here's a site I've found enlightening in many ways and I post it here for those who are not aware of it. Enjoy! www.quartzpage.de/#yep, just confused me more www.quartzpage.de/flint.html
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Post by Pat on Oct 29, 2018 17:23:57 GMT -5
gemfeller thanks! That is helpful. He is also on mindat.org
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