It does look like glass of some kind. Slag glass, I believe, is a recovered byproduct from like steel production or something. Moldavite is natural glass of volcanic origin, and sometimes that stuff is reconstituted and processed somehow to create a crisper looking gem.
So, both being glass, their characteristics are nearly the same. There is no crystal structure; the pieces are amorphous. They have vitreous (literally glass-like) concave fracture (the circular surface chips that look like how a pop bottle chips. They can have gas bubbles and other inclusions trapped inside. They are warm to the touch.
I'm not immediately sure how to make the call between the two. The refractive index is probably overlapping and there is no flat surface to make that test anyway. There are probably indications by the type of inclusions and/or an indication from a spectroscope that would be how one would identify.
For me, just based on the photo alone, it's a coin flip at this point. I might lean towards moldavite because it seems perhaps a little more likely that someone would mount up a freeform natural piece like this and use it as jewelry...but if it was available where someone worked as a byproduct, I can see how they might mount that up as a conversation piece too.
Sorry, but that's about all I got; it's not something we dealt with very often.
