Obsidian or volcanic rock?

[Bradley Morrison]

I found this rock near a quarry in Wisconsin. It is very close to Lake Michigan. It weighs about 0.3 pounds and it has a black-dark green color to it. In some areas in is very shiny like glass but in others it is almost like concrete. It is not slag and it is not magnetic. The dollar coin is there for size. It is rather hard.

 

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[tamrock]

Well it does look like Obsidian rock to me, but slag can look like Obsidian also. People find slag in just about everywhere a Railroad went, because the slag heaps produced by the steel refineries was a great source of aggregate for railroad ballast and that crushed slag got spread for miles across the country.

 

[Back-of-the-boat]

To me it looks like glass slag just my opinion.

 

[ToddsPoint]

It looks like slag. Also, obsidian IS volcanic rock. Gary

 

[Plug N Play]

We need to see a picture with it held against a bright light.

This is very generalized, and there is the possibility of overlap, here ...
But the best indicator I've found to distinguish the glass from the obsidian is to go by the color that shows when it's held against a bright light.

 

[Bradley Morrison]

There are some pictures of it right next to a bright light.

 

[Digger RJ]

Looks like obsidian to me.

 

[Red-Coat]

I see one enormous vesicle and numerous smaller ones. Not obsidian, but man-made glass (slag or frit).

 

[Plug N Play]

First, the aside. I'm speaking of colors, and we are currently in the middle of barbecuing California. I apologize. The sun is a weird red disc in the smoke, and you may see ashes in the pics. Colors look off a bit in my pics because of this.

The black green color you mention shows up in none of your pictures.
That's what I'd go by is the color, as a way of trying to separate green obsidian from glass.

We're not going to be able to determine this, but I took some pictures to help you study your piece.
Let's limit this to two terms ... opaque-we can't see light through it ... and translucent-we can see light through it, no matter how little.

"Against" a light we should see a hint of green/gold, if the obsidian has any translucency and can be described as green. (my pic is poor, it helps in person because you can wiggle it around) (This is obsidian with a hint of gold/green)



I've never seen obsidian in the bright colors I spoke of earlier. If I see bright green, either opaque or translucent, I tend to lean towards glass. This is glass.



Vesicles mean nothing. If man can melt it, let's not underestimate the power of volcanoes.
This is opaque green obsidian. Volcanoes have no problem making vesicles.



This compares translucent glass which has colors I've never seen in any obsidian. (rainbow obsidian is a whole different thing, yours isn't, that's an entirely different issue and we can eliminate that, I believe) The blue/green piece is actually opaque green, but still exhibits the brightness I'm stressing in the color.



Anyway, it's the type of GREEN I go by as the biggest clue, and, so far, we haven't seen any green.
You're going to have to interpret the green you think you see and determine if it falls into a bright green color as I'm trying to suggest, or does it show a hint of gold/green around the thinnest edges when held "against" a light.
That, I believe will be your biggest clue, while not being definite.

If you're up to it, break off a shard of this rock and you'll have a very thin piece that tapers to nothing. Backlight it and see if it gives you any more clues as to color.

As shown in your pictures, I lean towards slag glass (mostly because of locale) ... but would never exclude obsidian without further investigation.