FMF - Friends of Minerals Forum, discussion and message board

[thedude1994]

ok i found a strange stone/mineral a few years ago and i have no idea what it is.I found it on the pavement in my town.The town is in the foothills of a mountain . At first i thought it was a piece of asphalt or something because it was black and shiny but when i scraped it on a piece of paper it didn't leave any black traces.Then i noticed that under sunlight the edges became semi translucent and weren't black but a yellowish brown.The stone's surface is a bit strange it looks like the edges of broken glass but isn't sharp . It also has some white dots and stripes on it but those might be paint traces or something from the pavement . So what could that be?Is it obsidian
P.S. I found it in Sofia, Bulgaria that's near the Vitosha mountain and the thing is there are no places from which it could have come in Vitosha.

[Pete Modreski]

This is most likely some type of slag, a glassy waste product from a mine smelter. The glassier types of slag can look very much like obsidian.

[thedude1994]

[Jordi Fabre]

[thedude1994]

[Riccardo Modanesi]

HI to everybody!
But for the yellowish-brown edges disclosed by Theduda, I would agree with you: it looks like obsidian with crystobalite spheres inside (the greyish-white ones we all see in the photo). What kind of rocks are there in the Vitosha Mountains? Volcanic, plutonic, sedimentary or methamorphic? Obsidian is typical for volcanic rocks, it's very common in Sicily for example. If it is artificial glas, I would be so curious by knowing what kind of minerals are the greyish spheres inside!
Greetings from Italy by Riccardo.

[thedude1994]

[Josele]

Hello everybody,
I think there is a simple way to know if it is natural or man-made: with very few exceptions, obsidian has no bubbles inside. Look at it with a x10 lens, if you find many bubbles then is an anthropic glass.

[Pete Modreski]

That's certainly correct, about looking for bubbles.
One can never be absolutely sure about the origin of a piece of rock by just looking at a photo, or sometimes even from looking at the real specimen. But the brownish color, the small white sphere-like inclusions (these can often be seen in slag or glass; remnants of incompletely melted material mixed into it, or sometimes, the beginnings of crystallization of the cooling molten material), and PERHAPS (this is a guess) the overall shape of this piece seems to suggest a curved shape, like the shape of a crucible into which molten liquid may have been poured--all of these suggest, most likely, an artificial slag or glass.
Even more suggestive is simply the location in Bulgaria. Volcanic rocks there are much older, than, say, in Italy, and obsidian is generally found only in very young volcanic rocks. I looked up a bit about the geology of the Vitosha Mountains; there is syenite and other igneous rocks, intruding older andesite lavas; but, the age of these rocks is given as Upper Cretaceous--which means around 65-100 million years old, too old to likely contain fresh-looking obsidian.
There are so many ways that people may have transported and dropped a piece of artificial material, especially in or around any sort of town, and Vitosha is not very far from Sofia, where any sort of material might have been produced. ---Sincerely, Pete

[thedude1994]

[Riccardo Modanesi]

HI Josele and hi to everybody!
Oh yes, natural glass (including obsidian) HAS bubbles inside! Think about moldavite (the green one, found in the Czech Republic) or Libyan desert glass: they are both full of gas bubbles inside! Obsidian is not an exception: normally these bubbles are filled by greyish cristobalite micro-crystals, very common in SIcilian volcanic rocks, for example! What Pete says is very interesting about the geological environment: Bulgarian volcanic rocks are too old for containing obsidian. I just have to repeat what I always say: I just saw one or two photos and I just see the appearance of a specimen in the photos.
My suggestion is: take a loupe 10X or a small microscope and see whether the bubbles are full or empty! If full, it's probably obsidian, if empty, it's probably slag.
Greetings from Italy by Riccardo.

[Josele]

Thedude1994, you must belive Pete and Riccardo, really more experienced than me.
What I meant was that the air bubbles in obsidian are usually very small and barely visible to the naked eye or with a x10 lens, while in a glass slag are usually larger and can be seen easily. This is not a definitive identification system but it works in most cases to recognize a slag.

[Chris Foltz]

I have tri-flow obsidian from Washington State with bubbles in it...

[supertxango]

Hi all !

Probably obsidian has bubbles, but I think that the specimen shown by thedude is slag. I attach a very similar slag specimen collected two weeks ago at a Spanish famous minning area, La Union in Murcia.

Thanks and regards

[thedude1994]

[supertxango]

Hi, in my specimen craters or bubbles are bigger, but I have seen similar slags with smaller bubbles or in some cases difficult to see. Also probably your specimen edges will be less sharp due to erosion. Perhaps if you hit and crash your specimen (not please :-) you will get similar sharpen edges.

Regards.