Author Topic: What is this dark rock, found in Michigan? (Read 1145 times)

Sue · **on:** 12/10/2012 16:39:44

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?

RD · **Reply #1 on:** 12/10/2012 17:09:53

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

Sue · **Reply #2 on:** 12/10/2012 19:02:07

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?

RD · **Reply #3 on:** 12/10/2012 20:23:10

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 ]

Quote from: Sue on 12/10/2012 19:02:07

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

Sue · **Reply #4 on:** 12/10/2012 21:07:32

Now THAT'S funny! Thanks for the replies anyway!

Bill S · **Reply #5 on:** 13/10/2012 20:02:59

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.

Bill S · **Reply #6 on:** 13/10/2012 20:53:32

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.

Sue · **Reply #7 on:** 14/10/2012 13:59:58

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?

Bill S · **Reply #8 on:** 14/10/2012 14:35:14

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.

Sue · **Reply #9 on:** 14/10/2012 17:12:32

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.

OokieWonderslug · **Reply #10 on:** 17/10/2012 02:34:12

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.

Bill S · **Reply #11 on:** 21/10/2012 18:17:20

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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Author Topic: What is this dark rock, found in Michigan? (Read 1145 times)