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Jewelry, Gems, & Minerals/Please help me identify this rock

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Rock Found
Rock Found  

Same rock
Same rock  
Hi.

I live in South Africa and am a big rock collector. I found this rock in a field close to where I live. Just some background info. I live about 40 km away from The Vredefort Meteorite impact crater (http://en.wikipedia.org/wiki/Vredefort_crater).

I found it lying ON the ground and not deep down.

Both the layers are very hard (like dolerite, not shale or sandstone. The black layer is extremely smooth while the brown/red layer is more course (although still very smooth)

We do not have many systems in place in this country to identify rocks and I was hoping if you could please help me. I would greatly appreciate it.



Thank you in advance.

Answer
As you know, identifying rocks or minerals from descriptions or pictures can be a bit tricky, if not impossible. Sometimes characteristics can be gleaned, however, that narrow the playing field substantially.

At first glance your sample looks like a river transported rock (Did you find it within the vicinity of the Vaal river?), most likely made of chert and banded jasper (red). One unique feature is a small fault line through your specimen visible on picture two.

The rock is most likely precambrian and the banded features maybe very early stromatolites, remnants of one of the earliest colonizing life forms on earth.

The dark stuff might be dolostone. To verify you need to powder a small amount of the black stuff (very small amount, just a pinch) and put some acid on it. Strong vinegar will work but not as good. The powder will start to fizz and bubble with acid if it is dolostone.

If the black material is chert you will NOT be able to scratch its surface with a piece of glass.

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Uwe Richard Kackstaetter, Ph.D. (Dr.K)

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I can answer questions concerning minerals, mineralogy, gems, metals, and anything that has to do with geology. However, I am NOT a jeweler. Questions about values, settings, gem stone cuts and appraisals are best directed to other experts on this site. I can however aide in the identification of unknown mineral materials. As a public service and part as training for new geoscientists, our university department provides FREE mineral identification for individuals. Please contact me for details or go to http://college.earthscienceeducation.net/MIN/MINID.pdf for details..

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I am a professor of applied geology and mineralogy with many hours of field experience. Furthermore, I enjoy recreational gold prospecting and mineral collecting. As a professor I am engaged in research concerning minerals and their occurrence.

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Member of the GPAA (Gold Prospectors Association of America) as well as the Association of Environmental Geochemists. Member of the GSA (Geologic Society of America) Member of the AIPG (American Institute of Professional Geologists)

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Here is a small sampling: Mineral-rock handbook: Rapid-easy mineral-rock determination : written for anyone interested in minerals and rocks - Proctor, Peterson, and Kackstaetter;Macmillan Pub. Co. (New York and Toronto and New York) Physical Geology Laboratory e-Manual [CD-ROM], Kackstaetter, Earth Science Education LLC Colorado Front Range Self-guided Geology Field Trips, Kackstaetter, http://www.scribd.com/doc/27175290/Colorado-Front-Range-Self-Guided-Geology-Field-Trips

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Ph.D. in Applied Geology and Mineralogy. I am actively teaching courses in mineralogy and a variety of field courses with mineral collecting opportunities. Background in precious metal exploration.

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