jero
off to a rocking start
Member since September 2009
Posts: 11
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Post by jero on Oct 12, 2009 21:01:48 GMT -5
From the Chain of Rocks gravel bar, Mississippi River at Granite City, IL (north of St. Louis, MO) Not wet, polished, or even washed! Group 1 Group 2 Group 3 Any help appreciated! Later, jeri
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MikeS
fully equipped rock polisher
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Post by MikeS on Oct 12, 2009 21:04:23 GMT -5
A nice assortment of agates, jasper and chert it looks like to me...
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jero
off to a rocking start
Member since September 2009
Posts: 11
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Post by jero on Oct 15, 2009 15:54:16 GMT -5
Thanks Mikes. So, um, which one is which? I'm especially interested in knowing what rock tumbles down a river and shines like glass, when it's dry! What you listed are all types of silica, right? So, I'm wondering why Quartz from a river looks frosted and the 1st one doesn't? Later, jeri
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MikeS
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Post by MikeS on Oct 15, 2009 18:16:12 GMT -5
The first picture appears to be agate...pictures #2 and #3 appear to be agate as well (some people call the clear red agate carnelian) Group 2 appears to be chert or flint, and group 3 appears to be chert or jasper. The distinction between the three is somewhat hazy, they are all closely related materials, and yes, they are all forms of microcrystalline quartz.
The appearance a rock takes from being in a river varies depending on many factors, but silt in the river tends to round and polish them over time...
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jero
off to a rocking start
Member since September 2009
Posts: 11
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Post by jero on Oct 15, 2009 21:47:07 GMT -5
Thanks again! I decided to look up the definitions of agate, chert, flint, and jasper... This is what I found at geology.com/ (I'm sure there are places)... Agate is a translucent variety of microcrystalline quartz. It is used as a semi-precious stone when it is of desirable quality and color. Agate generally forms by the deposition of silica from ground water in the cavities of igneous rocks. The agate deposits in concentric layers around the walls of the cavity or in horizontal layers building up from the bottom of the cavity. Chert is a microcrystalline or cryptocrystalline sedimentary rock material composed of silicon dioxide (SiO2). It occurs as nodules, concretionary masses and as layered deposits. Chert breaks with a conchoidal fracture, often producing very sharp edges. Early people took advantage of how chert breaks and used it to fashion cutting tools and weapons. Dark varieties of chert are common and are often called " flint". Jasper: A variety of colored chert, typically red or green and often found in association with iron ores. Jasper is frequently used as a gemstone or in the production of ornaments. Agate is translucent and chert isn't. But, it appears chert, flint, and jasper are only differentiated by color? Later, jeri
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MikeS
fully equipped rock polisher
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Post by MikeS on Oct 15, 2009 22:26:09 GMT -5
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Post by roswelljero on Oct 16, 2009 20:35:09 GMT -5
I read through that thread... Good discussion, especially the part about 'why does it matter what it is?'... LOL I'll just go back to 'I'm not really sure, but I think it's...'.
Later, jeri
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MikeS
fully equipped rock polisher
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Post by MikeS on Oct 16, 2009 22:08:44 GMT -5
yeah, they are all pretty similar materials
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rockhard
starting to spend too much on rocks
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Post by rockhard on Jan 22, 2010 19:49:40 GMT -5
'round here (and lots of other places), if it's pretty or interesting, and fairly opaque, it's jasper. I figure that's in lapidary parlance, leaving the technical differences to the geologists.
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