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First Post, Black Specimen
  
  Index -> FOR BEGINNERS: What is it? Where is it from?
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danielgoldberg86




Joined: 04 Sep 2019
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PostPosted: Sep 04, 2019 14:54    Post subject: First Post, Black Specimen  

We recently built a house in Western Wisconsin about 400 yards from the St. Croix river. A friend of mine came over and we found a 3 inch agate in a pile of dirt excavated for our house, and I've been looking for more interesting finds ever since.

I am new to identification, and I am having a hard time figuring out this black specimen. In anything but bright light, it appears jet black. However when I brought it to work and looked at it under a microscope with bright light, it almost looked gray with flecks of black in it. In the bright light it also looked like it might have a semi-opaqueness, but nothing like the agate I found.

It is quite hard, I have other agates and when I tried to use a sharp point to scratch it, I could not (no effect to the agate either though, so that might not be a helpful test!) A metal staple did nothing to the rock either.

Obsidian was the first thing that came to mind (not glassy enough to be onyx? Or do I have those switched...), but I'm new to this so it's tricky to know.

The other thing about this region is that a lot of stuff was brought here by glaciers, so it can be difficult figuring out exactly where everything is from!

Thanks for any help, I greatly appreciate it as I get more and more into minerals.

-Daniel



IMG_6357.JPG
 Mineral: Obsidian/Onyx
 Description:
USA / Wisconsin / Western Wisconsin - St. Croix River Valley
1.5x1.5 inches
Black specimen, possibly obsidian? In bright light.
 Viewed:  533 Time(s)

IMG_6357.JPG


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danielgoldberg86




Joined: 04 Sep 2019
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PostPosted: Sep 04, 2019 14:57    Post subject: Re: First Post, Black Specimen  

Here is the same specimen, not in direct sunlight (and yes, rookie mistake I hit submit instead of add attachment so I had to enter a new post!).


IMG_6359.JPG
 Mineral: Obsidian/Onyx
 Description:
USA / Wisconsin / Western Wisconsin - St. Croix River Valley
1.5x1.5 inches
Obsidian/Onyx - Not direct sunlight
 Viewed:  531 Time(s)

IMG_6359.JPG


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Carles Millan
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Joined: 05 May 2007
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Location: Catalonia


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PostPosted: Sep 04, 2019 14:59    Post subject: Re: First Post, Black Specimen  

We understand your desire to know what your samples are, but it is too difficult to determine this from photos. If you have a rock shop nearby, or a lapidary club, we would suggest you go there to ask. In case for some reason you can't join a club then you can also try this other web page: https://www.reddit.com/r/whatsthisrock You can also read Asking: "What is this? / Where is it from?" and provide us with some basic information about the specimen.

We want to help those who are truly curious, but people must demonstrate their curiosity by providing us with key information before we can help them.

Sorry to be so strict but we receive too many queries from people asking more or less similar questions and finally we are a bit overwhelmed by it.

Thanks for understanding.
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danielgoldberg86




Joined: 04 Sep 2019
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PostPosted: Sep 04, 2019 15:07    Post subject: Re: First Post, Black Specimen  

Maybe you shouldn't have a 'Beginners: What is it? Where is it from?" Forum then?

I read the first post on instructions for posting and tried to provide all the suggested information (hardness/location/appearance notes/etc) If you are overwhelmed by the posts in this section...maybe don't view them? Or don't respond and let people who are interested in responding to beginners reply?
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alfredo
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PostPosted: Sep 04, 2019 16:02    Post subject: Re: First Post, Black Specimen  

Daniel, I‘m pretty sure your black rock is one of the many forms of silica, either a chert or jasper, and not obsidian. (Obsidian can only exist in geologically relatively young volcanic areas, as it devitrifies with age, and there aren‘t any volcanic rocks young enough east of the Rocky Mts.)
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danielgoldberg86




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PostPosted: Sep 04, 2019 16:09    Post subject: Re: First Post, Black Specimen  

Thanks so much Alfredo!

When I've typed in 'Black Rock' for the past month all the results have been about obsidian, onyx, or black tourmaline.

I figured it wasn't tourmaline or onyx, which left obsidian. But your reply helps me know I'm not even in the right ballpark! Chert or jasper make a lot of sense. I have a nice grapefruit sized breccia rock full of white and red chert and jasper. (See pic)



IMG_6350.JPG
 Mineral: Breccia
 Description:
USA / Wisconsin / Western Wisconsin - St. Croix River Valley4 x 4 inches
Breccia with red and white chert/jasper. Found in gravel put down in my new driveway.
 Viewed:  471 Time(s)

IMG_6350.JPG


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Josele




Joined: 10 Apr 2012
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Location: Tarifa, Spain


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PostPosted: Sep 04, 2019 17:23    Post subject: Re: First Post, Black Specimen  

The black one seems a kind of chert / flint that can be used as touchstone to check precious metals.
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danielgoldberg86




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PostPosted: Sep 05, 2019 10:04    Post subject: Re: First Post, Black Specimen  

I was able to take a picture of the specimen under a microscope at work today. Notice the black flecks amongst the mostly gray...substrate? Near the cracks it appears semi-opaque with a brown hue.


rock hdr.jpg
 Mineral: chert / jasper
 Description:
USA / Wisconsin / Western Wisconsin - St. Croix River Valley
1.5x1.5 inches
Black Specimen under Microscope
 Viewed:  171 Time(s)

rock hdr.jpg


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Tom Tucker




Joined: 03 Dec 2009
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PostPosted: Sep 05, 2019 10:48    Post subject: Re: First Post, Black Specimen  

Slag from an old charcoal iron furnace. A large furnace was located in Ashland - not all that far away.
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