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Author Topic: What is this dark rock, found in Michigan?  (Read 4634 times)

Offline Sue

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What is this dark rock, found in Michigan?
« on: 12/10/2012 17:39:44 »
I was given this rock that had been in a relative's garden for decades.  I've always called it the mystery rock.  I know little to nothing about identifying rocks and was unable to find anything similar. 
It was found in western Michigan very near water.  It is not magnetic, and I can't scratch it with a sharp knife.  It is incredibly heavy, but then again, this grandma doesn't usually lug around rocks of this size, which is just about square and covers about 3/4 of the seat of a kitchen chair.  I have no idea where to get acid to test it. The top of the rock and the other dark parts are colored green, and it almost looks like some sort of glass.   
The pictures, in order (if I did this correctly), are the top of the rock, the side of the rock, and the bottom of the rock. 
Any ideas?
« Last Edit: 17/10/2012 08:43:02 by chris »


 

Offline RD

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Re: Never saw a rock like this
« Reply #1 on: 12/10/2012 18:09:53 »
the green stuff looks like olivine ...



http://www.b u z z l e.com/articles/olivine-uses.html


attached to grey basalt ...


http://serc.carleton.edu/NAGTWorkshops/mineralogy/index.html

« Last Edit: 12/10/2012 18:51:14 by RD »
 

Offline Sue

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Re: Never saw a rock like this
« Reply #2 on: 12/10/2012 20:02:07 »
I forgot to mention that when I shine a strong light on it, it looks almost like it's sweating. 
Is there a way to determine if it is indeed olivine?  Would it be worth anything, or should
I crack it in half to make it easier to carry and toss it into the garden for decoration?
 

Offline RD

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Re: Never saw a rock like this
« Reply #3 on: 12/10/2012 21:23:10 »
the gem-quality version of olivine is called peridot ... http://www.collectorfinejewelry.com/buyers_guide_peridot.htm

[ gem-quality would be transparent, like green glass ]


Would it be worth anything, or should I crack it in half to make it easier to carry and toss it into the garden for decoration?

If there a few small fragments you could try selling them to the healing-crystal-energy nutters who have a dirty aura or constipated chakras :) ...

Quote
This stone is [Peridot] known as "the healers stone" as it helps in aiding people who work in healing others to cleanse their auras and release neutralizing the toxins on all levels.

Peridot purifies the subtle bodies and mind. It opens cleanses and activates the heart and solar plexus chakra.
http://www.lightandlovereiki.com/Crystals/crystals_peridot.html
« Last Edit: 12/10/2012 21:45:31 by RD »
 

Offline Sue

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Re: Never saw a rock like this
« Reply #4 on: 12/10/2012 22:07:32 »
Now THAT'S funny! Thanks for the replies anyway! 
 

Offline Bill S

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Re: Never saw a rock like this
« Reply #5 on: 13/10/2012 21:02:59 »
Hi Sue.  On a more serious note, I would hesitate to say anything that might lead to the destruction of a nice large specimen, but yours does look a bit weathered, judging by the very different colours of the various facets.  If it were mine I would be tempted to do sufficient violence to it to reveal a fresh surface.  I have a couple more thoughts; will post again when time permits.
 

Offline Bill S

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Re: Never saw a rock like this
« Reply #6 on: 13/10/2012 21:53:32 »
Quote from: Sue
I forgot to mention that when I shine a strong light on it, it looks almost like it's sweating.

If RD is right about the olivine, we are looking at an igneous rock, towards the basic end of the range, so it probably contains plagioclase feldspar.  Plagioclase can undergo a process known as sausuritisation.  The appearance of sausuritic Plag. is often described as "cheesy".  The only examples I have seen have had a rather dull lustre, but I believe it can also have the "sweaty" look you mention.

As far as getting a definite identification for the olivine is concerned; colour is a good start, it needs to be light to mid green.  Hardness is about 6.5 (Moh's Scale), but it can be as hard as 7, so it is going to be difficult, or impossible, to scratch with a piece of quartz.  However, if you can scratch it, it should show a white streak.
 

Offline Sue

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Re: Never saw a rock like this
« Reply #7 on: 14/10/2012 14:59:58 »
Thanks for responding Bill.  I was able to scratch it, and it did leave a white mark where I'd scratched.  I was able to get a small piece off the dark green top fairly easily, using side cutting pliers. I used a laser light to see if the light would shine through it, and it does.  I wouldn't call it light to medium green though.  It's pretty dark.  I decided to shine a bright flashlight on the side and bottom, and the green, while slightly lighter than what is on top, glowed a neon green.  So I guess for now I'm not going to crack it in half. 
I've read different artcles on olivine/peridot, and I have come across people who say to use filters, special lights and acid, but I'm pretty clueless about how to do any of that.  Should I try to find a local rock shop to see what they think? 
 

Offline Bill S

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Re: Never saw a rock like this
« Reply #8 on: 14/10/2012 15:35:14 »
Sue, a rock shop might be able to help, but if you are within range of a museum with a good geological department, that would be much better.   Back in the days when I was actively collecting I had some valuable help from the Geological Museum in London.
 

Offline Sue

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Re: Never saw a rock like this
« Reply #9 on: 14/10/2012 18:12:32 »
That is great advice!  I live in the Chicago area, and I'd bet the Field Museum, or perhaps the Museum of Science and Industry would have someone available.  I am going to have to try and figure out how I'd get the rock there because of it's weight. 
 

Offline OokieWonderslug

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Re: Never saw a rock like this
« Reply #10 on: 17/10/2012 03:34:12 »
That looks like alloy slag. There is a smelter of some kind in WV in a town called alloy. It produces rock that looks just like that. They dump it in the river or it finds it's way there. I have several pieces of it stashed somewhere. Michigan is an industrial state and I suspect that what you have there is slag from a smelter.
 

Offline Bill S

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Re: What is this dark rock, found in Michigan?
« Reply #11 on: 21/10/2012 19:17:20 »
Sue, it is, of course, quite possible that Ookie might be right (spoil-sport), but I hope you will let us know if you succeed in getting a geologist to look at it. 

Many years ago, I found, on a mine dump in Cornwall, what I thought was a specimen of native iron; only later did I discover that it was the unrecognisable remains of the old mine machinary. 
 

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Re: What is this dark rock, found in Michigan?
« Reply #11 on: 21/10/2012 19:17:20 »

 

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