Ok I’m not the best to answer this but I see there are no comments so I’ll kinda give a stab at helping.

What you may be describing is either slag or a vesicular (term for bubbles) rock and maybe there’s some sort of mineral, perhaps filling one of the vesicles but I’m not quite sure based on your description.

Now if this is a rock and not slag, vesicular rocks are typically lava and I’m gonna say without being much into lapidary that it might not take a polish well. Especially if it is really jagged as you say.

It sounds like doing something like throwing it into a tumbler might not be what you’re looking for. Other options are cutting it with something like a rock saw or a wet tile saw and grinding down or off unwanted portions. Lapidary people usually have specialized grinding machines but I’ve seen people do it by clamping down a sander and taking the rock to it or by clamping the rock and taking something like a dremel to it. If you just want to polish the mineral and keep all of the spikey bubbly bits it depends on the mineral and its hardness but dental equipment with the grinders/pastes and the polishers might be worth considering.

There’s plenty of resources on YouTube or just out in the world on how people do this so I suggest kinda poking around and using the term “lapidary”. Rock and gem clubs are all over the place too.

Hope that’s kinda what you’re looking for. Oh, also if it is slag it might polish depending on what it was, so there could be hope but I uh…wouldn’t trust it to not have someone toxic up in there if you say it might be a little metallic.

Pictures help in these kinds of situations and again I’d seek out lapidary people because they have knowledge about the prep side of displaying a cool rock and often have geologic knowledge too if you’re curious about that.