FMF - Friends of Minerals Forum, discussion and message board

[Robson Vieira]

Hi folks. I was walking around in a small park near home and I found two lustrous stuff like a tree root. Obviously, I have investigated the material and I picked it up.
The images below show the material.
They are like a glass material, and one of them has a poor rainbow hue.
Honestly, I think it is a slag, but I would appreciate your most qualified opinion.
The area I found the piece is a basaltic one, where I have found some vesicular basalt as well.
Thanks,
Robson vieira

Colina, SP, Brazil

[Peter Megaw]

Slag...bubble in glass

[Robson Vieira]

Hi Peter and thank you for your comment. I would like to know how to differentiate slag from natural glass or another natural material. Could you help me? Is there a kind of rule to see?

[Pete Modreski]

Hi, Robson,
This is a different Pete answering you, but let me try.
There isn't any one simple answer to what you ask, only some general guidelines. Mostly, it's a matter of experience and familiarity with these kinds of materials.

The only common type of natural glass is obsidian, and it is found only in areas with recent volcanic activity. And because obsidian, compared to many rocks and minerals, is not too hard, and is brittle, and easily worn away by erosion, it is rare to find it as transported pebbles in an stream deposit that is not immediately in a volcanic area. Most obsidian is black and, of course, glassy, but it grades into grayish, cloudy, and "pitchy" looking but not glassy material (perlite). And of course, there are always exceptions--some can be greenish, but that is very rare.

There is also natural glass in the form of tektites, but those are very rare, and only found in certain areas where tektites are common (i.e., Thailand-Vietnam).

There are also "fulgurites", melted soil or rock (usually, sandy soil) from lightning strikes. These are usually grayish in color, very full of bubbles, and quite fragile and easily broken. Fulgurites are also quite rare.

Slag is usually cloudy, not fully transparent glass; often banded to some degree; usually containing a lot of bubbles; and tends to be quite varied/irregular and non-homogenous in its appearance and texture. It can range considerably in color, from greenish to gray to nearly black. Some slag, that is very rich in iron content, can be nearly black, opaque, and will feel unusually heavy. Most slag will have surface features that look obviously like molten liquid has been poured and solidified.

One also finds pieces of solid, clear or partly clear glass, in any color, leftover fragments from a glass-making operation. These usually look distinctly like simply "a broken chunk of colored glass".

I think those are about the best guidelines of advice that I can offer.

Pete Modreski

[Robson Vieira]

Hi Pete. I learned a lot with your explanation. Exactly what I had needed. Thank you.

[Volkmar Stingl]

[alfredo]

Robson, there are many different types of slag. In several different places in Bolivia I used to find slag that looked like yours - It came from small lime kilns that were widely scattered over the countryside for calcining limestone to get lime. The slag contains melted ash from the wood and brush used as fuel, melted residues from the limestone, and melted clay from the adobe bricks used to build the kiln. Unlike slags from smelting metals, there is usually not much iron in it, so it could be nonmagnetic (unlike slag from smelters, which is commonly magnetic).