Unpopular opinion: Obsidian isn't always volcanic glass...

Mark Moore · 2026-03-30T22:30:03.369683+00:00

I keep seeing people at rock shows call any black, shiny rock 'obsidian', but a lot of it is actually slag from old iron furnaces. How do you all tell the difference in the field?

troy_ross5mo ago

My cousin bought a whole bucket of "obsidian" at a flea market last year, turned out to be 100% industrial slag. The real giveaway is that conchoidal fracture, it looks like broken glass with those curved ripples. Slag just looks chunky and wrong, like someone melted a bunch of junk together. I always tell people if it looks like it came from a factory and not a volcano, you probably have a nice piece of trash.

8

taragibson5mo ago

Reminds me of people calling every clear rock a diamond lol.

5

ray_king5mo ago

Yeah I used to grab any black shiny piece thinking it was obsidian. The big thing for me now is looking for bubbles or a weird texture, slag usually has those. What's your best tip for telling them apart?

1