what are these?!

[ojete1977]

I am a completely ignorant about rocks but a friend of mine got hold a bunch of them and we are trying to figure out what they are. Here is the link to the video I've made.

 

[ModernMiner]

I am a completely ignorant about rocks but a friend of mine got hold a bunch of them and we are trying to figure out what they are. Here is the link to the video I've made.

I’m guessing slag.

“
stony waste matter separated from metals during the smelting or refining of ore.
"the burning liquid iron was forming a scum of slag"